I have a XFX 8800GT the ones with stock core speeds of 600Mhz.
Your getting realy poor FPS, first let me give my specs

PSU 550Watt Corsair
CPU AMD Athlon X2 5400+ Black Edition stock @2.8Ghz now @3.1Ghz
Ram 4GB Buffelo Firestixs DDR2 800
Mobo MSI K9N-SLI V2

Its cased to a Tacens Aluminum Exreme
My Mobo is Sli but i use only one Videocard and I get around 25-35 FPS.
Crysis is a system killer but you should be able to get atleast 20 FPS Average.