Stay away from department store bikes. You will get garbage components on a heavy cheaply built and poorly welded frame being put together by some minimum wage stock boy that doesn't care and when it breaks there is no one going to bail you out except for your wallet.

Stick to bigger name bikes (Trek, Specialized, Giant, C'Dale just to name a couple) These will be built with higher quality drive components that will have a better chance of lasting you with out breaking. If you go the new bike route go to a LBS (local bike store) and build a relationship with them. You might pay a little more up front but this usually comes with free tuneups and cheaper/free repairs. Also you might find that they have a quality used bike that you could get for pennies on the dollar. Spending a little more time and possible a little more money now will make things a lot cheaper in the long run. Just a couple things to think about.