It's hard to recommend a good used bike since there are so many possible models and so few available in any given area. See what you can find on craigslist in your size and reference it by looking up the model on bikepedia.com, then researching the parts if you don't know what's good. Feel free to ask about models or components in this thread (although I do not recommend linking directly to an ad in case it's a killer deal and some member snaps it up).

As mentioned above, you just won't find a new bike in that price range that's worth riding. Also, let us know what type of riding you plan on doing. Do you want to ride smooth fire roads? Bumpy singletrack? Rocky terrain with mad jumps and drops?