The key to success is to push the video card as it is better than most $500-$600 computers out there...but DON'T push that cheezy 17" monitor...that is so...2001-2002.
I just checked out Frys.com and they have a similar computer to yours, brand new, but with a 4000+ Dual Core AMD64, for $459.00 after rebates...but with a cheezy ass Nvidia 6100 graphics card.
I'd try to price it at $700/obo and see what you pull. Don't expect much at that price point, if you get nothing or no offers or anything, then drop to $650/obo...and keep doing so until it sells. I bet you could probably pull $550-$650 on craigslist.