After experimenting with different SLI and graphics settings for 2 hours, I found the absolute best performance optimization for Crysis under Vista is to dump DirectX 10.0 altogether.
This occurs by default in XP as you can't get DirectX 10.0 for XP at all. However, if you are in Vista you have to force a DirectX 9.0 override via the commandline.
To do this, just create a shortcut on your desktop like so (you file path may vary):
"%Program Files%\Electronic Arts\Crytek\Crysis\Bin32\Crysis.exe" -dx9
You can have the best of both worlds (for the best performance and image quality) with the help of this config file hack which allows Windows XP to run on the "Very high settings" mode found at http://www.ngohq.com/news/12492-crysis-demo-very-high-settings-hack.html. The very high setting is not normally available in the Game->System menu in DirectX 9/Windows XP.
With these settings, I can run Crysis on the max setting normally available in DX 10 (Very High) on a Dell XPS 1730 at 1024x768 with 2x AA very smoothly. I've found these settings to be the best to play with... you can tackle a crowd of North Koreans at close range without a stutter, and then stop to admire the view. What's more, you get all the volumetric light effects normally reserved for the DirectX 10 only crowd.