Im also considering an upgraded computer in the next month or so. I would suggest just getting a desk top without the monitor if your monitor is a decent size. Dell has some good deals, look in the on-line discussion section. Also, you can look at the Dell Outlet section (just watch for the shipping costs).
If you are erious getting a deal, you shouldn't be spending more than $700-800 (and that should include a 19in flat panel monitor).
Happy shopping