problems with pc resetting/ crashing

i have built a pc recently and am having when running wow and star wars galaxies, sometimes the screen just freezes for 60secs then picks back up again and sometimes (usually every 40mins) the pc just turns off and resets- i am racking my brains as to what the problem is, i have reciepts for all the components so if i can find out the problem with a fault can return. current setup is:-

amd athlon 3200+
1.5gb unnamed ram
80gb hdd
500w power supply supllied with case
leadtek winfast a400 tdh unclocked
gigabyte k7 triton motherboard model ga-7n400e-l
i also have recntly tried installing an aero 7 cpu fan which has reduced cpu from 63 deg to 53
currently i have case opened and have tried room fan blowing on system but still crashing so dont think it is overheating.
i have 66.93 nvida drivers and have tried 71.22 and still same problem and some blocky colours in game so changed back to 66.93
i have not overclocked.
please help as this is draining my patience.
dazz

Do you have a fan for your games/graphics card?

only the built in fan on the graphics card (gforce 6800).
dazz