For 600 to 700 bucks, you are going to find a bike the rides fine, but will have a lot of low end components. The cheaper bikewill be a bit heavier than a more costly bike and may not perform as nicely (shifting, etc.). As far as brands names go, it doesn't really matter as the major brands will all have low end and high end stuff.

If you really want to get the most for that price range, you might consider a used bike. If you know someone knowledgeable in bike that can help you in the buying process, you can get a pretty decent bike for for about $600.