More RAM will definitely help.
Better GPU will definitely help.
Better CPU will help.
All things considered though, GPU is the way to go. 8800GTS 320mb. Your CPU might bottleneck it a bit, but depending on your resolution you play at, it won't be too much. The performance jump to the 8800 will far outweigh any bottlenecks alot.
Lesson on CPU/GPU for Gaming 101:
-average CPU + great GPU = great gaming
-great CPU + average GPU = average gaming
New graphics card FTW! Just make sure your PSU can handle it though.