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)
Either you need to take a look at airflow issues in your PC case (it may not be taking in enough cool air for the GPU cooler to work efficiently), or you need to alter the fan speed settings on your GPU.
RivaTuner if I recall, will allow you to change the 'default' or automatic fan controls such that you can tell it to be at 50% at 50C, and 100% at 70C... or whatever works best for you. Could be as simple as upping the fan speed on the video card itself.