i know on the 8800GTS 320mb evga they have been selling for about $200-225 if thats because of the 8800GT. thats what prices i have seen on the hardforums for the current time. if you the 8800GTS you should call evga and ask them if you can do step-up with that card to a 8800GT. if you can.
I think that your computer sounds decent, just that your cpu is lacking. I might try to invest in a new cpu. That might knock it up in price, and might turn the tables on wether or not you can sell it. but I guess value would be at 700 usd