The "best buy" in bikes are mountain bikes in the $400 price range. Right now, many shops are selling their 2008 mountain bikes for 20% off. These bikes are tougher and more durable than road bikes that cost twice as much. That is why so many downtown bike messengers use them...they only get paid for what they deliver, not for time trying to fix their bike.
Visit every shop within a mile or two of your house. Buy from the shop that seems most committed to customer service...all bikes need service. A good bike shop can make a $400 bike feel like a $1,000 bike. Discount stores can make riding a bike a terrible experience...which is why industry sources say discount store bikes are used, on average, for less than 100 miles of riding before they are junked.