Overheating? I have been using an ATI Sapphire Radeon X800 for a couple of years now with no

Question

I have been using an ATI Sapphire Radeon X800 for a couple of years now with no problems. Just the last few days all my games seem to run for a about 5mins and then I get 'blocky' artifacts onscreen. Shortly after this starts I get a complete PC shutdown. The PC runs fine otherwise.
I suspect the card is overheating. Anyway to check this? If this is the cause, is it possible to replace the fan.

Any help or advice gratefully received....