vista prem 64bit
EDIMAX EW-7128G PCI Wireless Card - Retail
2 CORSAIR 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)
PC Power & Cooling Silencer PPCS750QBL 750W
Rosewill RCR-102 52-in-1 USB 2.0 3.5" Internal Card Reader
SAMSUNG 20X DVD±R DVD Burner
ASUS EAH4850/HTDI/512M Radeon HD 4850 with aftermarket cooler
msi 4850 with the same cooler as the other
Seagate Barracuda 7200.11 ST3500320AS 500GB
ViewSonic Optiquest Series Q241wb Black 24"
XIGMATEK HDT-S1283 120mm Rifle CPU Cooler
given the numbers in the custom build world have been crazier than a turbo honda, it is as bad as the past with maintaining value. Given my 11 years of custom pc ... I would pay the same for a 3400 prescott and pata 100 as the q9450. Of course vista and a dx10 card for the agp would be the big difference...making it the same. browse around and take the hit. It aint worth much. Altho, as I said, prescott HT and afterwrads could run for several more years even for demanding users...quite realistically.
If you plan to sell it, simply total up the current cost of the components (they are all still on the market) from Newegg. There is some depreciation for it being a year old, but that is offset by the labor and troubleshooting of building it. If someone is buying a prebuilt system they are willing to pay for the trouble of not building it themselves.
I haven't totaled everything exactly, but doing the quick math in my head of the CURRENT prices I think you are probably in the $1000 range....you really can't include the OS on a used system, since most people looking to buy will already have an OS and won't include that in their budget, or will be shopping for a name brand PC that will come with license also. You could probably add some value by overclocking it slightly if you are capable of that, since most people buying a prebuilt on ebay don't have the knowledge to do that themselves, just be sure to save the OC settings in the bios and be prepared to walk the new owner through restoring them since you will probably lose those setting during shipping.