Our test results show that a pair of GeForce 6600 GTs in SLI can rival the performance of a single GeForce 6800 GT card in some cases. That's not a bad upgrade proposition: add together two $200 video cards to get the performance of one $400 video card. Nor can the sheer suaveness of having two graphics cards in one's PC be denied. Nothing quite says "dead serious" like a pair of graphics cards peering out from behind that case window.

That said, the value proposition for dual GeForce 6600 GT cards isn't as stellar as it could be. SLI doesn't offer 2X the performance with dual cards, and in some cases, it may not provide any performance advantage at all. More importantly, the 6600 GTs in SLI often run into trouble at higher resolutions with lots of antialiasing and texture filtering enabled—precisely the kinds of scenarios where SLI should shine. They are, quite likely, bumping up against the effective 128MB video RAM limit. 128MB of video RAM isn't much for a high-end graphics solution these days, and that dynamic is only going to get worse as new games with larger textures arrive. Because of this limitation, I'd rather have a single GeForce 6800 GT card with 256MB of RAM than a pair of GeForce 6600 GTs.