i recently overclocked, my aim was for faster video encoding, but youll mainly be overclocking your gpu, i have done this and the difference is kina minimal, my ati 4870 was overclocked for a while, made a little differance, went up about 6-8 frames in some cases, so its small, but noticable for games like crysis, it made it playable.
More info about your hardware would help, in some cases it's totally normal to overclock, yet in some cases it can result in even worse performance, so it mainly depends on your hardware, computer type and cooling.