My computer spec

[Sephiroth]

Here is my system spec;

CPU: AMD 64 3500+ running at 2420 Mhz (A.I. overclock 10%)
Mother board: Asus A8N-E (Nforce 4 chipset)
RAM: 2X Kingston KVR400X64C3A/1G (1 gig)
Power supply: 350W Antec smart power
Video card: Nvidia 7950 GT OC (PCIE)
Hard disk: Maxtor 60 gig at 7200 RPM (IDE)
Monitor: Samsung 19' wide screen SyncMaster 941bw (LCD using digital plug)

So far, I could run every thing at maximum display possible untill I played Command and Conquer 3: Tiberium War. That game is CPU hungry. When alot of unit are attaking, the game seem to be slower and alot of lag apear. I was wondering what could slow down my system ? I was thinking about my hard disk but I was wondering how much a faster hard disk could improve gaming ? On another side, my power supply is only 350W but there again, I was wondering how much a bigger power supply could get gaming better ? Beside, I dont have anything fancy to power, only a DVD rom, video card and CPU.

Any suggestion or comment will be apreciated