I've had an old 12 speed, steel framed Miele (c. 1982) move me around Atlanta really well (Northlake area to downtown/midtown/Decatur). The roads in Atlanta are pretty good so my tires at 1.25 in wide, while thick for a racing bike, were plenty thin for commuting and still offered a comfortable ride.
Since it does rain a lot in Atlanta, and often suddenly, I had fenders, which if you're commuting are a must. I'd carry my work clothes in a back pack (which with the fenders would not get wet) but would much rather have had a pannier.
Performance Bikes (85/N. Druid Hills) I believe even has road bikes (drop bars, light aluminum frame, >18 gears) right in your price range. I'm looking at the Fuji Newest 3.0 for my girlfriend It is a great intro bike especially for what your after and it's only 550 $. Performance is also across the street from REI so you can ride bikes at both in one day easily and then right down the road is Bicycle South.