I say if you going to be include the Display you get a fair 700-800. Im not really sure if it worth selling for even that price it for you. I mean your going to be spending more than 800 bucks to get another pc later on right?
I'd say go for 800 on the computer. It's got a display, blue ray drive, good graphics card (?), 4 GB of ram, but could always be upgraded.
And I'm guessing it's custom built?
If you include all of those things in the description, you could probably sell it for 600-800.