Don't OC unless you get an aftermarket cooler or turn the card's stock cooler way up (creating a leaf blower). These card already run hot and if you're trying to hit 700/1800 you have a very good chance of cooking it. Maybe not immediately but it will not live very long.
Keep in mind that the memory is NOT cooled with the stock cooler. If you remove it you'll see that there's little foam pads between the metal base of the cooler and each memory chip. These do not transfer heat, they're only there to keet it from damaging the memory during installation as far as I can tell. If you're OCing 300MHz+, they are going to take some damage long-term.
That said, I don't feel a need to OC my 1800XT 256mb. I run Oblivion with nearly all settings at high, HDR at 1440 X 900 and it's plenty smooth. If you're getting 15-35 fps now, OCing to 700/1800 is not going to get you any higher than about 18-40. Not worth it.