My home PC is an Athlon XP 2100+ and I've used the stock cpu fan and casing fan and left the computer running 24/7 for a few months and no problems. (I am running SETI*HOME kasi).

But I did encounter a few hangs when I tried playing Generals (or other high resource game) around noon time. My room gets pretty hot kasi. So I upgraded the cpu and casing fan with GlobalWin and Delta unit. The GlobalWin has its own thermal sensor which acts as a throttle, the hotter the sensor reading, the faster the fan. The Delta casing fan moves 4 times more air but its very very noisy. I have a speed control on it so I can slow it down when the system isn't running games and similar stuff.

I even have two digital thermometers... to measure the cpu and video temperatures...

BTW, my shop's newest PCs are Athlon XP 2000+ with GF FX5200 video cards & 256DDR. Although not cutting edge speed, they still rock! And each system only costs around P17200 (no monitor). Hopefully these systems will run the next generation of games...