OK, I dual booted Vista and XP. Here is what I got for Crysis in XP:
DX9 SLI, 800x600, AA=No AA, Vsync=Disabled, 32 bit test, FullScreen, Medium Settings, No O/C, Win XP
2007-12-15 23:04:43 - crysis
Frames: 1963 - Time: 35106ms - Avg: 55.916 - Min: 35 - Max: 79
DX9 SLI 1900x1200, AA=No AA, Vsync=Disabled, 32 bit test, FullScreen, Medium Settings, No O/C, Win XP
2007-12-15 23:05:58 - crysis
Frames: 1974 - Time: 86735ms - Avg: 22.758 - Min: 18 - Max: 29
For comparison purposes, here is what I got with Vista:
DX9 SLI, 800x600, AA=No AA, Vsync=Disabled, 32 bit test, FullScreen, Medium Settings, No O/C
2007-12-10 21:40:37 - Crysis
Frames: 1939 - Time: 39425ms - Avg: 49.181 - Min: 28 - Max: 72
DX9 SLI 1900x1200, AA=No AA, Vsync=Disabled, 32 bit test, FullScreen, Medium Settings, No O/C
2007-12-10 21:42:04 - Crysis
Frames: 1966 - Time: 97822ms - Avg: 20.097 - Min: 14 - Max: 28
Looks like a 10% penalty with Vista.