as general rule of thumb for myself in buying/selling used parts, I value it at whatever 1/2 the new price of the parts are at time of sale, so assuming system is in good running condition, you are probably looking at $400-600ish
I think it also depends on how you want to sell it, and how fast you want to sell it. Obviously something like craigs list would be easier, because there is no shipping, but I would think you would be able to get more from it on ebay, but its not as quick and you would have to ship it. also, if you are willing to sit on it for a while you could set the price a little higher and wait for the right person.
the other option would be to upgrade it, as monitor, ram, psu, hdd (and possibly case) are probably all still good, and you probably could get a little more life out of graphics card. so for, say, $400 you could get a nice mobo and ivy bridge, which will make a hell of a difference I suspect
either way, you probably aren't looking at getting too much money =[
Honestly your gear is not that outdated. Just get a new cpu like the 8350 and a new gpu and you would be good to go. You could get a ssd too.
Well I have around $1600 to spend on a new PC, so I really don't want to keep this one. I already have some parts like 16GB ram, 750W PSU keyboard, mouse, headset, case, and I still have 1600 dollars left to spend, so I might get the GTX780 and the i5 4670k. But I'm not sure yet.