I won't disagree with the idea of buying used, but if you're rather new to guns and don't know what to look for, buying used may be a bit daunting.
In that price range you could likely afford a new Mossberg SA-20.
http://mossberg.com/products/default.as ... n=productsI've heard a lot of good things about the Mossberg 930 (their 12 gauge semi-auto). My dad uses a 930 and shoots well with it. It's functioned perfectly for him even though he didn't break it in properly and maintenance isn't high on his priority list.
I haven't heard a lot (good or bad) about the SA-20, but I'd look into it if I were in your shoes. I've seen it priced from about $450-500.
...On a side note, my first shotgun was a 12 gauge, and 12's are all I'd ever used until I got my first 20 gauge this past summer. It's a fun gun to shoot and very nice to carry for long hunting strolls. Ultimately, I'm glad to have both gauges, but I think you'll be happy starting with a 20.