Specs:
Intel Core 2 Duo E8400 @ 3.0 Ghz
4Gb of DDR2 800
ATI 4850 GFX Card
Seagate Barracuda 320Gb Hard Drive
Asus P5Q-Pro P45 Motherboard
ZeroTherm 120mm heatsink
Corsair 750W Power Supply (best of the best)
Antec 900 Case
Windows Vista Ultimate 64-bit (runs beautifully)
Microsoft Office 2007
Microsoft Works
Note: If you going to say nothing because it is windows DON"T POST
This is a tough one, because you can find alot of Quad core 2.6 Intel's with 4 gig ddr3,1333 ram, and 500 sata's gig drives w/450- 500watt or better power supplies for about 6-700 maybe better if you look hard. It's incredible the deals you'd find. If MS Office is the Ultimate package that would maybe give you a 100+ or so to the rig only if the software is included. "Take a look at this page on Tiger direct to get an idea."
Given that the Mobo hardware and the ATI card is on the upscale though, and the case is not a cheap one, I think you could pull 6-700 possibly on Ebay. Maybe better if you include a monitor.
I guess the key here too is "remember" and take it from one whom has sold many used ones. As soon as it leaves the lot, it's value decreases by almost 40%. If you get someone who really wants to barter, could you give any indication as to how many hours the power supply has run. How many hours the hard drive has worked. This is all considered wear and tear, and savvy buyers will ask these questions. If you cannot answer them, you can only hope to sell to people whom do not know better, and yes there are plenty of those out there too.