First thing to note is that all graphics cards have slightly different max overclocks. I have two GTX 260's, one of which can be stable at 735 and the other max out at 702.
You can probably get to 700MHz without a problem with your card, but some people are unlucky and can only push to say 650, others might have freaks and get to 780MHz without any issues at all.
Best thing to do is to overclock as you would a CPU...
Increase clock speeds a little at a time, each time testing for a good 30-60 minutes in furmark and other benchmark tools to make sure the overclock is stable. Keep pushing the card until you get artifacts or your computer crashes, then dial the card back some until stable again.
Once you think you've hit your limit, leave a benchmark tool such as furmark running for a good few hours to make sure your temps and stability is okay.