http://www.slickdeals.net/#p5434Dell Business has the PowerEdge SC420 P4-2.8GHz PCI-E server system for $289 + tax w/ free shipping. You must login to see free shipping.
The system has an 8x PCI-E slot and no AGP slot. Someone was able to cut the plastic separator on the mobo and install a 16x PCI-E Nvidia 6600GT card in the slot.
Click here.
Click on the "Outrageous Deals" near the top right.
Customize the PowerEdge SC420 for $279.
Select the 80GB Serial ATA Hard Drive for $10.
Specs:
FREE UPGRADE to Intel Pentium4 processor, 520, 2.8GHz,1MB Cache
256MB DDR2, 400MHz,1X256, Single Ranked DIMMs
80GB, Serial ATA, 1 inch, 7.2K RPM, Hard Drive
Network Card Onboard
48X, Compact Disk Drive, 680M I, Half Height, Black
Onboard SATA, 1 Drive connected to Onboard SATA Controller
1 Year Basic Warranty Plan
Although this is marketed as a server, it loads up windows xp/etc just fine.
I picked up its predecessor, the poweredge sc400 a few months back and am VERY impressed with its performance. This is a great deal if you have an older pc you are looking to upgrade as you can throw your hard drives and cd/dvd burner into this one, plug it into your existing moniter, etc. and have a nice, fast, new pc for cheap.
Also, note that you can add a keyboard and mouse for free.