Crysis no brakes

Hi all. Help to decide what I should upgrade.
System:
CPU: DualCore Intel Pentium D 920 (2800 MHz; 14*200)
Motherboard: ASUS P5LD2-SE
Chipset motherboard: Intel Lakeport i945P
RAM: Samsung 2*512MB (DDR2-533 DDR2 SDRAM)Timing:5-4-4-11
Video card: NVIDIA GeForce 7600 GT (256 Mb)
HDD: WDC WD2500JS-00NCB1 (250Gb)
The power unit on 350 Watt

Crysis default are exhibited the lowest system requirements. I play: resolution - 1024*768, additional settings on high., only quality shaders on medium. The RESULT - 20-25 fps, but there are times when the computer hangs, is loaded and then plays(quite often). Want to enjoy the beauty without being distracted by the brakes. Thought to add two more strips 512 are identical to my own. Waiting for your advice.
PS: a Friend told me that my system is not playing, is this true?!
P. S. S.: And it makes sense to put the 8800 GTX or the processor will not allow?
I am waiting for advice, THANKS in advance!