I used to have the same card and amount of memory, but my cpu was a Dual core E6550. Try these settings if your playing the first crysis
Everything on high, shadows, post processing on med
1024x768
No AA
I used the Very high hacks on crysis, and managed some settings at very high, but in the later levels such as the snow levels and ship level, it struggled so i had to turn some stuff down. Another option is to try the CCC mod.
http://www.crysis-online.com/forum/index.php/topic,29717.0.html
I wish this mod was out when i had my 7900gto!!.
Warhead is supposed to run better than the crysis, so you can probably run higher settings.