The numbers speak for themselves: ATI is the DirectX9 king. Dual Radeon X1900 cards in CrossFire took 30 out of the 40 possible wins against GeForce 7950GX2 Quad SLI. Pixel shader intensive games were 10 to 20 frames per second faster, and in brute force games and situations the Radeon platform did well. When Nvidia was able to utilize special rendering modes, the four graphics processors showed their muscle, as was the case in F.E.A.R. However, if there is no special application or rendering pathway that yields performance gains, the four processors of Quad SLI fall behind two processors under CrossFire.
I would be amiss if I did not talk about SLI vs. CrossFire. SLI is easy to set up and has been around in the same format for as long as it has existed. CrossFire has had its ups and downs, and is not as friendly to set up. While its performance is best under DirectX9, those looking to implement a system on their own can go through quite a bit of fun getting CrossFire to initialize correctly on its initial installation.
We ran into some problems with CrossFire on this system, and had to reinitialize CrossFire. This can throw numbers off, and you need to cross-test to make sure the numbers are accurate and replicable. We changed the main output of the cards from CrossFire to single card, to single card on the second card, and back to CrossFire - we cycled through all of the settings to make sure CrossFire was "happy." Once the numbers were aligned and able to be reproduced over and over, we finished the testing.
So if you find yourself getting frame rates below what you expected, mess around with CrossFire and it will work. It is not always this difficult, though, and ATI's new dongle-less implementation makes CrossFire even more user friendly. Still, we will not see this on all models of an ATI processor generation until the R600 comes out.
iBuypower's 2006 Dream Quad-SLI Gaming System