We love hardware. As users ourselves we feel your pangs of wanting the newest, fastest rig and components. Unfortunately it's hard to justify the cost of bleeding-edge hardware at the best of times - more so with the cost of single graphics cards at times exceeding the $1000 price barrier. This is as much as some users would spend on a total system upgrade.
SLI (Scalable Link Interface) therefore is a big ask of customer's wallets, requiring the purchase of not only the appropriate motherboard for connecting two graphics cards, but two identical cards at that. As NVIDIA's entry level SLI capable model, the 6600GT has an approximate street price of $400; you're looking at well over $1000 for two cards and motherboard before you even look at the cost of RAM, hard disks or the 500-550W power supply recommended by NVIDIA to even attempt running a pair of 6800 Ultras in SLI.
Curious to see exactly how much performance gain a second physical graphics card provides we grabbed an SLI capable motherboard (in this case the matching ASUS A8N-SLI Deluxe), whacked in 512MB of PC3200 DDR, an AMD 3500+ socket 939 CPU and off we went. The set up is fairly simple, the only tricky part being remembering to flip the PCI-Express graphics controller card nestled between the slots from single card to SLI before plugging in both cards. Having small fingers might also help.
Given the constantly changing nature of the graphics industry, particularly on the software development side, we've opted to use Beta drivers in the form of 67.66 ForceWare drivers for testing SLI capabilities.
As there are only two ways SLI can render images, SFR (Split Frame Rendering) and AFR (Alternate Frame Rendering) and both have distinct advantages for different types of scene rendering. NVIDIA is manually compiling profiles for games as they appear but as we go to print only a handful of titles have these profiles available. Four of these are synthetic benchmarking utilities, then there's a game still on the drawing board and one in demo form.
If you can stomach the cost though, there's plenty of performance to be had here. As the graphs above clearly attest to, running a pair of cards in this mode can yield results of almost two times its single brother without the need for anything more than the two cards and the SLI bridge which ships with the motherboard. The good news here is the ability to purchase the second identical card later down the track when the budget permits.
Anisotropic filtering and full-scene antialiasing remains the domain of top shelf graphics hardware; the most notable performances coming when testing with four samples enabled. While SLI shows its true prowess in each of the synthetic benchmarks, real world performance was slightly different, with Half-Life 2's alternate frame rendering method in SLI taking a massive hit to single card operation in our custom demo. While the losses are 5 average frames per second at most, we're surprised at the loss in performance, most likely due to the need for further render optimisations and the card's workload sharing. Doom 3 turns the tables again, putting five frames over SLI set up in the default resolution but losing 13 and 17 frames with AA enabled and pushing the resolution to 1600x1200.We suspect these anomalies are due to driver maturity, we wait eagerly on the 70 series drivers, hopefully by which time the list of SLI supported games and applications will have boosted and the performance issues we've experienced have been rectified.
Until this happens, SLI is still just a proof-of-concept technology, and when its stability is rock solid it will no doubt be widely supported. As it is, this is some high performance tech, but only if you're willing to shell out the $2000 plus for the rig.