If it goes above 95c, I'd get worried. Personally 80-85c is too hot for me.
As for OC'ing the core, try another program. Have you tried OC'ing only ~20 mhz?
I just changed the GPU clock using "Rivatuner", and that worked.
Btw, what do you suggest i should do with my temperatures?
I wasnt playing a really heavy game (Left 4 dead 2, 1280x1024).
I am planning to buy a bigger monitor (24" 1920x1080), and play a higher resolution,
wont my GPU get to hot, and raise to like 95-100c?
At the moment i have Manual fan control disabled. (so, automatic fan control enabled)