FPS DROP during CS Gameplay, Help!

Hello, I need some assistance.

Problem: FPS drops significantly during gameplay when about 4+ players are on my screen and during heat of battle. When I am the only player in server, fps is fine.
fps drops from 100ish, to about 30 ish fps.

What is the root cause of this fps death in my pc?

PC Setup:
350 watt power supply
Striker Extreme m/b
P4 3.0 Ghz 800fsb
1 gig ram at 677 mhz
Nvidia 7800 GT

Thanks in advanced.