It depends on the card. But you probably can. Just keep going until it gets unstable, once it's unstable you can pull it back a little or up the voltage and keep going, just make sure it isn't getting to hot.
That is entirely up to your graphics card. No two cards will overclock exactly the same. If your card can remain stable and not over heat at a higher clock, then yes, you can get better performance. If not, then no.