I've never overclocked anything but I wanted try and overclock a graphics card..
I was thinking of buying a Radeon HD 7850 and overclocking it.. but is it possible to buy any HD 7850 card? Or the version of the card has to specially made for OCing?
Afaik some GPU`s are voltage locked, thus not allowing for proper overclock.
If you want to be sure regarding the possibility of overclocking, search for reviews on the card you are interested with to get more details about this.