I'd urge a little more caution than the other guys are suggesting. Not that their advice is wrong, but I've heard one or two horror stories from people who have been unlucky with their GTs, and found that they really had very little headroom for overclocking. One guy just boshed in a quite low overclock that someone else had attained, straight out of the box, without testing, and found his GT was not only incapable, but also fried as a result. His fault, but still, that kind of practice often works.
So (as you should with all overclocking), take it little by little, better to waste time and keep your GPU intact, particularly since these GTs seem to run hot and have a smaller OC margin.