DO NOT buy just any stick of RAM. Make sure it is at least a descent brand because cheap RAM is the cause of a large number of lock-ups and mystery crashes.
Good brands (in my book, there are others):
Mushkin, Corsair, Crucial, Geil, OCZ, Kingston(ok)
I have also have tried (after reading many positive reviews) the PDP Patriot RAM recently and like it (rock solid stable). I hope its ok to mention a website, but I really like newegg. com really competitive prices, fast shipping, excellent customer service.
If you are adding a 512 stick to the one you have, you probably should try to buy the same brand/model.
Otherwise, you may want to buy a dual pack of RAM. This is not simply for boards that support dual channel memory. Having a matched pair is always good for stability, although sometimes you can get lucky and mix sticks.