If your budget is only $800 I'd start looking at fixed gear bikes. You can get a decent fixy for under $800. Try the Giant Bowery, Specialized Langster, or Trek 5th district. Fixy's are pretty good commuters. That's why so many bike messengers use them. Not to mention that they are cheaper initially, and maintenance wise.
If you're going to buy anything else, you should get at least Shimano Ultegra (or comparable) gears. Anything less than that will be shredded in a year or two. Then you will be looking at a new bike or new gears, which will cost you almost as much.