Download rivatuner or coolbits
Once you install it, up the core speed and mem speed 5-10mhz, apply it, run 3dmark or whatever your favorite game for a while. If you have no artifacts, the dots and slashes all over the screen, downclock a little bit.
I would recommend is running 3dmark03/05 prior to you OC'ing then once you finish your OC, run 3dmark again and see what kind of improvement you get
GPU's are supposed to withstand up to 100C temps, so once it gets around there, i would be looking into some better cooling and some AS5
AIM/Xfire - pzerofgh