Help with hardware problem

This is my first post at tom's.
here i go:
when i am doing some graphics intensive process such as playing a game or doing some 3-D rendering, my system just crashes.
It sometimes happens during the prolonged usages of wondows aero too.
I have tried changing the power supply and the situation seems to be better but the only difference it makes is that the system crashes after some more time.
the temps on comp are pretty high with the GPU idling at 70 C
The CPU is idling around 80 C.
I am planning to reapply the thermal paste and buy a new heatsink.

Where do you think the problem lies?
My system specs are follows:
Q6600@2.4 GHz
3 GB ddr2
8500 GT
generic 500W power supply (couldnt find a branded one in my are.)