I pulled the trigger!
First, thanks for all of your help. I nearly went with a refurb'd HP (there's 2 really, really nice quad cores now @ $505 shipped - one at 8GB RAM/640G HD, the other at 6GB RAM/750G HD) and a ridiculously good deal @ $450 shipped on a Core 2 Duo.
In the end I ended up getting a "leftover" new Lenovo (thanks for that suggestion) for $550 shipped. It has an Intel Quad Core Q8300 @2.4Ghz, 8GB DDR2 RAM, DVD+- / CDRW writer, NVIDIA GeForce 9300, and a 1TB HD. It has Vista on it but I have 2 new copies of Windows 7 Professional (real ones in the box through my college) so I will fix that. I'll use the included keyboard and speakers and reuse my existing screen and laser mouse. Not a bad bit of hardware for the price, really.
So thanks for all of the help!

