I got a Dell Inspiron 530 a couple months ago so I could run my high-end mapping software. I thought it was a very good deal - 2.8ghz Core 2 Duo with 6GB memory and 750GB hard drive for about $600 at Best Buy. It's a very fast machine, but has the integrated (on-board) graphics card which makes it less desirable for games and video, which I don't need either on this machine. I don't know if you can still find these, but if you're SURE you don't need top graphics performance, the machines with integrated graphics will usually be cheaper.
FWIW, my understanding is that you will get a free upgrade to Windows 7 with any PC that you buy now. Of course, it probably makes sense to wait for a pre-installed version in order to spare yourself the agony of upgrading it yourself.
Also... you could consider switching to the Mac. You are going to have to learn new ways of doing things under Windows 7 anyway, so this might be a good opportunity. There are versions of the Mac Mini for $600 and $800. For another $100 or so, you could also install Windows XP on it and have everything. Just a thought.
http://store.apple.com/us/browse/home/shop_mac/family/mac_mini?mco=Nzk2MDUyNg