I play the standard Crysis. I don't have Warhead at this time, so I couldn't help you much here like I wish. Even though I can play Crysis on high on my 9500GS (which is probably considered inferior to 8800), the frame rating still isn't perfect, but it's tolerable at least for me, and I am not very picky on frames. Most of the time it's very smooth but at times, it can dip down. The next gamer might not accept the performance very well, I'll have to admit, but I can stand it.
What I think, your bottlenecks could be your CPU or RAM.