Soldato
The best tool i know of is GPU Z. You can click the number to make it track the maximum/minimum/average or you can just get it to show real time readings. You can also stretch the window and change the refresh rate to make it show the data over a fair long time to give you an idea of how it changes during a gaming session.
Didn't realise gpuz could do that! I'll mess about with it when I get round to playing something that'll push the card a bit, thanks