it depends on the card, and how much of a factory OC it comes with.
a vapor-x 5870 has alot of OCing headroom over the stock 20mhz boost.
the MSI cyclone SOC 4890 has virtually none over its stock 150mhz boost.
but if you dont try, you'll never find out
Good answer, I have a BFG 9800GTX+ OC 1G and it has hardly any headroom left and the little OC gain isn't worth messing with.
Also had 2 BFG 8800GT OC 512M that had plenty of headroom left, so OCing them was well worth the gain.
To the OP, you'll just have to test, and find out what you have and where you can go with it, and see if OCing it, is worth the gain.