I'm not too sure on how your graphics card performs exactly but I've read that the performance is somewhat similar to mine (8800GT 512MB). When I play, i run the game at 1680x1050 with no AA. I have 3 or 4 things turned up to Very High (which has now been renamed enthusiast) & for the most part I get around 30FPS. To achieve this I did actually overclock my graphics card by a bit & I had my CPU running at 3.2Ghz. But I'd reckon that your system should respond similarly to mine.
Should be fine.
I can run the original Crysis very well at 12*10 (4xAA) everything on max (only shadows and physics down a notch). I think Crysis will use all 4 cores of the 6600, plus the 9800GT is faster than the 8800GT (a bit)
ALSO, the engine for warhead has been optimized.
So long as you're not expecting 25x19 8xAA or something, you'll be LAUGHING!