Beth is debating if it's worth it to buy a "good" bike or a Wal-Mart bike. I can tell you with absolute certainty that getting the better one is worth it.
Here's the thing about cheap bikes: They're disposable. They don't last, they're not as rigid, they're extremely heavy and have cheap parts (and crappy seats). My Trek was something like $600, but it's going to last me 20 years as long as I clean it and get it overhauled every couple thousand miles. If you buy a decent aluminum bike, and they can be had for under $500, from a reputable bike shop, you'll get a ton out of it.
The Bianchi I got in high school, 1989, is still completely useful. I outgrew it only because it was heavier steel (typical back then), and I wanted 700c wheels.