As with all overclocking, you're likely to see a small performance boost, and whether this is worthwhile depends on how limiting you find the card's results now. If you find yourself thinking a 5 or 10% higher frame rate would make a game really playable, then it could be worth experimenting with. How much extra you can actually achieve only trying it will tell.
Potential downsides of overclocking include increased power consumption and hence heat output (with the chance of causing overheat damage), more chance of glitches like crashing or lockups.
If your PSU is adequately rated to supply good power to the card, and the cooling of card and system is sound (eg card fan and heatsink(s) clean and case well ventilated), overclocking could be satisfactory. Very few manufacturers release a card that is already very close to 100% of its rating, as the return rate in warranty would likely be higher. If the card is still within its warranty period, check the manufacturer's policy regarding overclocking, whether it voids your warranty.
Another thing that may count against overclocking potential, if the card only has basic spec cooling, is that modern video cards can selectively shut down internal functions to reduce power consumption/heat output. If the cooling of the card or the system case isn't adequate to support overclocking, you may find that the performance actually decreases when the card becomes hot after a period of operation.
Heh, Sterling just beat me to it...
Edited by Platypus, 04 December 2008 - 06:59 PM.