Costco has the Dell XPS One All-in-one 20" Desktop for just $599.99 for members (non-members pay $629.99). All in one pc's, are great options if you want a desktop that doesn't take up much space and has a clean look (fewer wires running behind your desk). Also, because it has an integrated tuner, you can use it as a TV.
The identical machine over at Dell will cost you $300 more, so you're getting a great price on this -- the lowest we've ever seen.
Note, some people have asked about the ease (or difficulty) of upgrading this machine. The good news is that the entire back of the XPS One can easily be removed for upgrading RAM or the Hard Drive.