With videcards, without quite a large overclock, you won't get that much of a boost, and might only get about 5-10 FPS here and there, but I guess if you do have a good make of card, this is worth doing as it can at least send a dodgy jumping game to a smooth game.
There are some programs for cards that can overclock your card pretty good, and will test the limits for you. For Nvidia cards, I've heard of Coolbits to be great at this.
What they basically do is shows a 3d image, and when it goes to far, you'll start to see artifacts (dots and 3d errors), and so you can then back track to find best overclock your card can get to without them:
Even though this takes a bit of time, its much better than trying to do it yourself, which might damage your card in the short run.
I hope this helps