using Coolbits registry tweak for nvidia or Rivatuner witch I believe works for both nvidia and ATI, but not 100% sure on that.
ATI has their own system in the advanced properties section
Right click desktop>properties>settings>advanced and somewhere in there
It's real easy to OC and as long as you don't leap it by a huge amount you'll be ok, and also make sure you watch your temps
By far the easiest component to OC without frying IMO