I recently built a system with a Athlon X2 4200+ and a Gigabyte Geforce 6800GT PCI-E on an Epox 9npa Ultra. I find that when I play Counter-Strike for an extensive period of time (read: more than two hours), I experience a gradual drop in framerates. Usually, I start out around 70-80fps and end up at a choppy 50-60fps. When the system is resting, the GPU temperature is usually around 62*C with the processor at around 40*C and the system temperature around 37*C. When the frame rates get choppy, the GPU heats up to 65*C. Is this too hot? Should I install a case-mounted fan next to the GPU to resolve this problem?
Temps look good. My 6800GT idles at 58C or so. Under load (1+ hour of gaming) it goes to around 64C. So, that is not your problem. Sounds like a memory leak to me. I had that problem with FarCry online. During single player mode, the game was fine. If I went online, I would start at like 45 FPS, and after 15 minutes it went down to 15 FPS. Updated when a new patch came out, and that fixed it. That was with my GeForce4 Ti4200 at the time. See if there is an updated patch available.