Graphics card is much more easier to overclock then a PCU.
A freeware RivaTuner does the trick.
Each card has Core clock, Shader clock and memory clock and in GeForce 9600GT for instance it’s 650/1625/900.
Currently I run it at 701/1752/1000 which gives much better FPS,
RivaTuner allows for monitoring the temperature, its best not to exceed 80C, however GeForce cards can handle up to 120C!!!
My card reaches 65C under load although if i push the specs a bit higher it occasionally crashes.
To test FPS use 3DMark 11 for latest cards and 3DMark 6,7 for older ones.