You can always 'fix' your CVars in XP too
This site is very detailed for the DIYer, it really takes forever this way
Or you can download this thing which is a CVars already set to go, you'll have to change the brightness, contrast, & gamma to fit your monitor though.
As for the card that can run this smoothly, both the 3870x2 & GX2 can @ 1920x1200 or less. The GTS can do it @ 1440x900. The 9600gt just don't have the power, or the shaders, to do it - SLI'd or not. & like DanishDevil said, always backup your CVars.
That CVars make Crysis look & feel better by loading all the textures into a cache instead of updating it every time you move, which can make even the fastest pc's go into permanent hibernation