@ OP, Dude its not worth more that 800 bucks. Just a friendly price suggestion. 9600GT is worth at max 1400 so your 8600GT is worth definitely lower than that. However you are free to sell it at whatever cost you wish.
Guys I have sold 8400gs and 4550 for 750 each just a few months ago. Dunno why you are asking a 8600gt for 800 bucks... Anyways OP drop down the price a little bit as the memory on it is quite less i.e 256mb, I had 512mb on both of them.
^ You must have got plain lucky Also as cards get older, the price difference between 2 different cards also diminishes. Say at the launch time the price difference between a 4850 and 4870 would be say 5 thousand. As years passed by if you want to buy both the cards second hand the price difference between them would hardly be 1000 or 1500 at max.So for really old second hand cards, a higher variant card fetches only few bucks more.
Here is a new thread in other forum. http://www.e(r)odov.com/forums/nvidia-8600gt-256mb-ddr3/44746.html , he is selling it at 1100 and still doesnt have any takers for it. I am pretty sure he will struck a deal at 800 bucks max after a few days. Search on TE itself and you will find a fella selling the card at 1400 last december. OP might be able to sell the card locally for bloated prices but on tech forums its very difficult. @OP, please dont take offence. My intention is not to hurt you anyway. How many offers have you got till now for the card ? None I guess. Makes sense cause at 2.1K second hand there are better cards out there. Get it. OP, do one thing. Consider yourself to a buyer for the card and someone is selling you this particular card. How much would you offer for the card ?
Btw , I had bought a 8600GT with warranty left for 2500 in year Oct' 2008 myself.