At 1024x768, the 8600 GT dominates the 2600 XT in this title. The strange thing is that the minimum frame rates stay relatively close.
As resolution increases, the race becomes tighter, but the GeForce still enjoys a comfortable lead over the 2600 XT. While both cards are technically playable at 1600x1200 with an average frame rate over 35 and a minimum frame rate over 20, most users would probably prefer the smoother 1280x960 resolution.
With 4xAA and 8xAF enabled, the only playable combination is the 8600 GT, which musters passable performance at 1024x768. The 2600 XT can only manage a minimum frame rate of 12 fps at 1024x768, which isn't something I'd consider playable.
While this is a solid victory for the 8600 GT, remember that the 2600 XT can muster a frame rate in the same neighborhood at 1600x1200 with no AA or AF enabled.