NVIDIA SLI Introduction
What is NVIDIA’s SLI technology all about? To put it simply, it is using two PCI-Express video cards in tandem instead of just one. This effectively “doubles” your gaming “performance,” or at least that is what it did for 3dfx back in the first days of SLI. We think that SLI today is more about providing you, the gamer, and a better overall quality of experience. Let’s face it, if speed is what you want, you don’t need a high-dollar video card for that. If what you are looking for is the most cinematic and immersive gaming experience you can get, then SLI may be right up your alley. Here today we are going to discuss upgrading options and what they may provide in hopes that our readers can make the right SLI decisions for themselves.
Obviously, upgrading to a SLI capable system needs to be well thought out. First off, moving down the SLI upgrade path means only using NVIDIA video cards, at least currently. It also means purchasing specialized SLI NVIDIA video cards and a special motherboard. For those of us not wanting to buy a complete SLI system now, today’s SLI configuration leaves us room to buy “One Card Now and One Later.” We think that most people will follow the “OCNOL” upgrade path and expand to SLI as prices of additional cards come down as the market moves forward.

The Motherboard
A NVIDIA SLI setup of course requires a special motherboard with two PCI-Express slots capable of accepting SLI video cards. While we will see SLI motherboards from the usual suspects, ASUS supplied the A8N-SLI motherboard for this guide. Our A8N-SLI uses a NVIDIA nForce4 chipset recently launched by NVIDIA.    
You can see above that the A8N-SLI leaves a nice space between the PCI-Express X16 video card slots for airflow. It also has a small card that clips into what is actually a PCIE mobile socket. This card is flipped around to allow the signals on the motherboard to spread out to two X8 PCI-Express sockets for SLI or to divert all that bandwidth to one X16 socket in single card usage mode. Of course both physical sockets are of the X16 spec, even though they are really operating at PCIE X8 speeds when in SLI mode.
In the last three pictures above, you can see the SLI connector that sits on top of the cards and connects to their “golden fingers.” This connection allows up to 10GB/sec transfer rate and allows the cards to “stay in sync.”
The Video Cards
Rest assured that while NVIDIA is currently the only SLI game in town, we will see a similar solution from ATI next year. So for all of our readers that bleed ATI-red blood, all hope are not lost, but for now NVIDIA has the market cornered.
But not all NVIDIA video cards support SLI. Currently the only announced products are the GeForce 6800 Ultra, the GeForce 6800 GT, and the GeForce 6600 GT. If you check NVIDIA’s website, it specifies that the “vanilla” 6800 will support SLI as well, but we are yet to be made aware of this by seeing functioning cards. We are promised that we will start seeing these PCI-Express cards showing up in retail in December, but be advised that it will really be Q1’05 before we see any great quantity of either SLI motherboards or SLI video cards on the shelves. Below is what two 6800 Ultras look like when poised in all their SLI glory.
Subsequently, while two Ultras take up a lot of room, two 6600 GTs are left with more than enough room to breath.

Video Card Compatibility
Of course there has been some enthusiast talk about just what cards will work together and what cards will not. First off you will need cards with two like GPUs. Meaning that SLI will require two 6800 Ultra, or two 6800GT, or two 6600GT video cards. We would also suggest that you look for the NVIDIA SLI Certification Logo. While we have not yet had a chance to test retail samples, cards showing the SLI certification logo even from different manufacturers should, in theory at least, work together in SLI configurations. Meaning that a BFGTech SLI certified 6800 Ultra should work with an Asus SLI certified 6800 Ultra. Of course that is yet to be seen by us, but NVIDIA has seemed very solid on this point. This should of course make moving to a SLI configuration over a long period of time a bit more comforting in that you can find a SLI video card to work with your first purchased card.
SLI Upgrade Guide Objectives
I am sure that some of you are wondering why we are calling this a “guide” instead of a “review”. There will be plenty of “reviews” today, so we wanted to cover this from a little bit different slant. Going to an SLI rig may be “no-brainer” decision for some of you but there are many gamers out there wondering exactly what it will deliver for their hard earned dollars. If you do make the call to build a SLI rig that will be upgraded with a second video card in the future, you will want to carefully choose what flavour of GPU you buy for your initial install. Meaning, do you start with a 6600GT, 6800GT or 6800 Ultra? If you are constrained by budget, obviously the 6600GT may the way to go for you, but then again, would saving your pennies for another couple of months in order to go with a 6800GT be a better value for you in the end? Or would going with a less expensive motherboard (SLI motherboard prices should be in the sub-$200 range once supply is solid.) with a single slot solution be better for you? For you ATI owners, we will also show you how your solutions stack up against NVIDIA’s.
Bottom line we hope to show whether or not SLI is right for you and if it is, what video card(s) would likely be a fit for your project.
Enabling SLI

With one video card operating in single card mode the ForceWare drivers detect that the card is capable of SLI and notifies you that you can in fact take advantage of the SLI setup with another SLI video card.

Once you have your two GeForce 6600GTs, 6800GTs or 6800Ultras you will need to reinstall the connector card on the motherboard so that the “dual video” side is inserted into the connector slot. In its default configuration it is set for Single Video Card operation. Turning it around enables SLI and sets both PCI-Express ports to PCI-E X8 operation. Do this step before you install the video cards to ensure you have room to work.

Next you must install both video cards in the PCI-Express slots and connect them together with the provided SLI bridge connector. This little piece of technology can move data at 10GB/sec between the cards.

After you have installed both video cards, connected the bridge together and enabled SLI operation in the motherboard BIOS you can go ahead and boot your machine. Clicking on the balloon pop up will bring you to the SLI configuration tab in the ForceWare drivers. From here there is only one check box needed to get SLI working. There is another check box which lets you select if you wish to view load balancing. This shows you in-game if it is using Split Frame Rendering (SFR) or Alternate Frame Rendering (AFR) and how much work each card is doing. After you select these options it will ask you to restart.
SFR and AFR describe how SLI is being done in a particular game. Most new games will allow SFR to be done. This allows both video cards to share the workload of any single frame shown on the screen. However, some games do not play friendly with SFR and there require AFR. With AFR, each card draws every other full frame. AFR is not as efficient as SFR in most cases, but does solve some issues when SFR does not output the desired image. Profiles are set up in the driver to help selection of the appropriate mode and should be invisible to the user. That said, it will be very likely that SLI users need to update their driver set as newer games are purchased. It will be interesting to see how this works out.

Then a balloon pops up telling you SLI has been enabled and that is all there is to it. SLI installation could not be much easier.

You will notice after you have flipped the card on the motherboard to “Dual Video Cards” the bus interface now reads as “PCI Express X8” indicting that your transfer rate is at X8 instead of X16 as it would be in single slot configuration.
Test Setup:
ASUS A8N-SLI Deluxe (nForce4), AMD Athlon 64 FX-53, 2 x 512MB Corsair XMS PC3200LLPro TwinX Dual Channel DDR400, Western Digital 74GB Raptor SATA/150, Windows XP Professional SP2 with DirectX 9.0c.
and
ABIT AV8 (VIA K8T800), AMD Athlon 64 FX-53, 2 X 512MB Corsair XMS PC3200LLPro TwinX Dual Channel DDR400, Western Digital 74GB Raptor SATA/150, Windows XP Professional SP2 with DirectX 9.0c.
EVGA GeForce 6600GT PCI-Express – Operating at default clock speeds 500MHz/1GHz using ForceWare 66.93.
NVIDIA GeForce 6800Ultra PCI-Express – Operating at default clock speeds 400MHz/1.10GHz using ForceWare 66.93.
NVIDIA GeForce 6800Ultra PCI-Express downclocked to 6800GT speeds at 350MHz/1GHz using ForceWare 66.93.
ATI Radeon X800XT-PE AGP – Operating at default clock speeds 520MHz/1.12GHz using Catalyst 4.12 Beta.
ATI Radeon X800Pro AGP – Operating at default clock speeds 475MHz/900MHz using Catalyst 4.12 Beta.
We ran all GeForce PCI-Express video cards on the ASUS A8N-SLI nForce4 platform in single card and dual card SLI configurations for this testing.
We ran into some issues getting PCI-Express ATI Radeon X800XT-PE and X800Pro video cards working on our SLI motherboard. Therefore we used an AGP Radeon X800XT-PE and AGP Radeon X800Pro on the ABIT AV8 platform with the same CPU and RAM. There is no difference in gameplay performance between the PCI-Express and AGP8X bus so this comparison does work.
At this time the only driver that is compatible with SLI is ForceWare 66.93, which is why we used that driver version instead of the Beta 67.02 available from NZone. We used the default control panel settings on each video card. For the ForceWare drivers this means Trilinear Optimization was ON, Anisotropic MIP filter optimization was OFF and Anisotropic sample Optimization was ON.
We used the default CATALYST A.I setting and disabled VSYNC.
We will look at gameplay evaluation performance in three of the top games of today, FarCry and Half Life 2 making comparisons.