I don't know much about this stuff and will ask here when I get mine, but I've been looking for the past 2 days at some deals. That is a good deal but a monitor is pricey. Add that and you're up to what you can get ALMOST for new. That is a good spec PC but no monitor and $600 used? hmmm I guess I'd agree with $500, I mean you can't really get it much less.
I say go for it, especially if you need one. You know he has'nt messed with it if he's your buddy.