Introduction

The announcement of SLI and the nForce 4 in the middle of 2004 generated a lot of talk in the hardware community. There were the skeptics who thought that it would remain a limited release product and be relegated to a niche solution but when pretty much every single major manufacturer is producing a SLI board it is hard to substantiate that claim. It is apparent that a product is making an impact in the mainstream segment - this past weekend when I walked into a store and overheard guys inquiring about SLI boards and when they can get a pair of video cards to go along with it. For those that are not familiar with SLI (Scalable Link Interface), the one line summary is that with certain PCIe motherboards (with dual x16 slots), a pair of identical GeForce 6 based video cards can be used in tandem to nearly double video performance - a more extensive look at SLI can be found here. This has several advantages - for the uncompromising hardcore enthusiast, this means they can take a performance leap into next-generation levels today. For mild-mannered Clark Kent and those of us that are constrained by a budget, this means that 6 months down the road when we are craving more performance, we do not need to shell out the big bucks for the ultra expensive high end card but can drop in another identical card to double performance.

Look for cards and motherboards with this logo to ensure compatibility

The Hardware

Currently, the only two chipsets that are SLI certified is NVIDIA's nForce 4 and Intel's Tumwater chipset. In our testing today, we will be looking at Gigabyte's GA-K8NXP-SLI and a pair of reference 6800GTs from NVIDIA along with Gigabyte's single slot SLI 6600GT solution in the 3D1 (review here).

It is important to note that SLI requires two of the same cards from the same manufacturer. This means that a 6800GT cannot be paired with a 6600GT. We have a reference 6600GT and a Gigabyte 6600GT that refuses to go into SLI mode even though the Gigabyte is just a reference board. Until there is a tool that allows a work around for this, end users should take care in choosing a card / brand that they are comfortable with as this is the only case where compatibility is more or less guaranteed.

The twins...

There are apparently several implementations of the motherboard SLI setup and the one from Gigabyte is the NVIDIA-approved solution. There is a bridge piece that looks sort of like a small stick of memory. One side disables the secondary PCIe graphics slot leaving the first slot in x16 mode. The second side splits the x16 lanes into a dual x8 configuration for SLI. The two graphics cards are connected with a small bridge piece (provided by the mainboard manufacturer).

Along with the mainboard and graphics card, a SLI rig should be paired up with a very fast processor - the almost doubling in performance only applies to situations where the game is graphics bound, not CPU bound. In a CPU bound situation there will be no gain in performance.

The final piece to the equation is a quality power supply. We've had a bunch of low end systems with cheap power supplies blow out and it decided to take no prisoners - everything inside the computer was dead. Power requirements for even a single 6800GT and 6800 Ultra are quite hefty already so do spend the money for a reputable PSU.