Well, using the typical used-is-50%-of-new rule, you'd be looking at about $1k for that machine. I doubt you could get much more for that by selling the parts.
Parts cost on ebay(buy it now):
Proc: $200
mobo: $130
Memory: $60
video card: $95
HD: $50
Monitor: ~125
KB+mouse: $25
Blu-ray: $40
Case: $50 (No clue, but it's a good guess.)
Windows - $0-100 (You aren't going to be able to sell it unless it's part of the whole machine)
Total cost of parts: 775+100.
Sell it for $850 and you'll definitely get a bid. You might get $1000 if you're lucky.
Also, please note that I'm not trying to lower the price to pick it up cheap; I have no money, and I already have a comparatively-spec'd machine, so I wouldn't be buying this machine(well, ok, for $100 I might, but not for anything reasonable.

)