I was in the same position back in January and bought a brand new single-speed from "bikesdirect" for $300. It was a pretty decent bike for the money and all nice and new. I was silly to buy a single-speed at 315lbs and eventually sold it this summer for the same money I paid! The buyer was thrilled.
However, they have many multi-speed bikes at your price point that seem like a great deal to see if you will be bitten by the bike bug or not. If you wish, you can always upgrade later and you havent lost a ton of $.
This one looks nice at your price:
http://www.bikesdirect.com/products/...e_record_x.htm
As I say, "bikesdirect" were both responsive and courteous and I got more than I expected for the low price point.
HTH, magohn