Originally Posted by Big Helmet
Assuming you find a decent shopworker to talk to and that you can test ride a few bikes, note for yourself the differences between your current rig and what they're offering. The new bike may feel lighter and more responsive. Work the brakes. The new ones may be more powerful, stopping the bike quicker, more quietly, and more securely. Work through all the gears; try to ride some gentle hills around the shop if possible. You may find the better bike's gears shift quicker and more precisely. You may find a bike that fits your body better, or that for whatever reason just speaks to you in some way that tells you you've got to have it.
The right bike shop is more important than the model of bike, as there is very little difference between similarly priced bikes. You can get a decent bike for less than $700 - see the tread at the top of the road bike forum, but for commuting you will also need a bike that can take a rack and fenders.