I use Rivatuner because it uses low-level overclocking, but tons of tools can do that. The reason why you can't overclock is proberly because you have a overclock disabled bios on the gfx. The tricky part about this is the the bios don't stop you from overclocking, it just crashes everything! You need to flash the bios with a unlocked one if you wanna overclock. Youd can everything you need to flash it here! ... and do make sure you get the corect bios!