I second the opinion above to get a quality used bike if you only have a few hundred bucks to spend, rather than any new bike from the racks of any big box discount store (Costco etc.). Good bikes can readily be repaired and last many years. Ask you local bike shop what they have in used bikes, and look for the good makes on Craigslist. Some standard good makes are Trek and Cannondale, but there are a lot of quality bikes out there. I have a Cannondale Super V that I paid $150 for, it is a mid 90s bike that cost $2500 new and had maybe fifty miles on it through Craigslist. You will also want a relationship with a local bike shop to keep any bike you get in tune.

One thing that no one has mentioned though, is that bikes really need to fit the rider. Go down to the local bike shop and find out what fits you. The fit of a bike to a rider is very important for comfort and power because the bike more or less becomes and extension of the body. The other thing is, even when you know the numbers for the bike size, usually a top tube length, don't assume it will still be the best bike for you because geometries vary and the same size in a differing geometry may be totally uncomfortable. I think this is why people get crazy about certain brands, they find their fit in a brand and do not want to try anything else. One way to reduce this conundrum is to try and find a seller with similar physical attributes as your body (tall with light build and long legs, etc.)