Download coolbits from here:
*www.overclockers.com.au/techstuff/a_coolbits/
To find out the extent to whic u can overclock, increase the gpu speed by 5mhz increments and run 3d mark and look for ANY graphic glitches. If anything is found, decrease by 10MHz. Do the same for vid mem clock.
I guess you just have to download the 3d mark coz it is the best for detecting overclock artifacts. Like everything in overclocking, this too will require patience
Maybe someone here knows of a smaller utility.