I really doubt you'll see $800 for it. You have an older chipset, DDR2 RAM, no OS, and small hard drives. Even though you say the 5850 is only a week old, nobody is going to believe that, unless you can prove it with a receipt.
I think you'll have a hard time seeing $600 for that. If I were you, I would list it on Ebay with a starting bid at $450, and see what happens. Parting it out and selling it is an option, but I think some of those parts are going to be harder sells than others.
You asked so do be pissed when truth hurts. Noobs are gonna be your only potential buyers, and they dont want the hassle of installing windows. Basically $450 starting bid and hope you luck out. Anyone that is willing to mess with installing a OS knows they can get a better PC for $800.