I'm mostly into mountain biking these days. For the price range you're talking about, have you considered getting a single-speed ride, like the Redline Monocog? I think new one's list for a little over $400, but you ought to be able to find something used and they're dead-stock reliable, hardly anything on them to break compared to a geared bike. With a single-speed you'll also get a nicer bike for the money. Craigslist is one source, the classifieds at
www.mtbr.com are another, there's ebay if you're careful, and it's well worth asking around at local bike shops. You can also search for websites for local bike clubs.
The main thing I recommend is staying away from department store bikes. Not only are the components extreme low-end, but too often the assembly is done by people without a clue. My father-in-law had a Mongoose full-suspension from WallyWorld that he'd had around for a few years before I knew him, when I saw it I immediately saw the suspension fork was on backwards.
I've even seen a K-Mart ad with the same thing in the picture. I've heard too many other horror stories, like light oil in bearings instead of grease, bolts never torqued down, etc. Plus you can run into real problems getting parts for the el-cheapo box store bikes and some shops won't even work on them.
BTW, if you're going to lock it up at college, having a good lock is a given but don't overlook other things, like getting rid of the quick-release on the seatpost and doing something to secure the wheels. There's a company that makes replacement quick-release skewers where the "handles" you have to turn are removeable, making them virtually impossible to take off even with wrenches until you insert the "key". Keep the bike inside when and where possible, especially at night. And don't go "Autopian" with the bike, you want it to look like something less desirable, so mud, electrical tape and watercolors are approriate. Just keep the moving parts in good shape and lubed.
Guess I have motorcycle on the brain. I have been out in the garage all day today working on mine. The heat must be getting to me 
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