Something seems out of place. I have a single 8800GT and can run it at a cross between gamer and enthusiast settings (i have a custom config, and my 8800GT is overclocked) and I am averaging 35-45 fps, sometime 25 during intense fighting.
I think i can help. I have spent a fair amount of time optimising Crysis. Here are some ways to improve your performance:
1.) Because the game lags during the fighting, i am wondering if it is your processor causing the bottleneck? (the video card doesn't seem to be a likely culprit). Find some way to monitor the load on your processor as your play. Vista is known for being a processor hog, and if you have alot of processes running in the background your processor could bottleneck alot. Try turinging down the physics effects in Crysis.
If you need help find ways to monitor the load on your processor while simulateouly playing Crysis, just ask. ^__^
2.) What screen resolution are you running at? Screen resolution KILL's Crysis! Try turning your monitor resolution down a notch (make sure to keep the currect aspect ratio). Personally, I would rather crank down the monitor resolution so i can crank up the AA. Turning the monitors resolution down 1 notch from my native resolution gives me 10-15 FPS more in Crysis!
3.) Custom config files can make a HUGE improvement in Crysis. There are hundreds of custum configs, which one works for you depends on your system and the look you want.. Just google "crysis warhead custom config" and you will get a gazillion results.
Here is a link to a few. Custom config can give you another 5-10 FPS.
4.) Make sure you have the latest Nvidia drivers, the latest version of direct X, and all of the updates for Crysis Warhead. Out of date drivers can cost you like 5-10 FPS. Also, defraging your hard drive can help the load times by about 10+ seconds shorter.
5.) I don't know if Crysis Warhead got any better than the orignal Crysis in the area, but...the orignal Crysis had way better performance on a 32 bit operating system then a 64 bit operating system. I think there is a way to run the game in 32 bit mode on a 64 bit OS. I don't know if that would help, but it is worth a try.
Oh, one last thing i remmbered....
The Nvidia controll pannel has this option where you can force better AA or AF filtering on everything. Once, I turned this option on to force AA in Masseffect. Then, I forgot that I forced AA on everything, and spend like 3 hours wondering why Supereme Commnder was running like crap. Make sure nothing is out of place in the Nivida Controll Pannel.