Pictures would help, because the amount of wear on the system is important.
If it has little wear that is noticeable, and no bad scratches and the like on it, I could see it going for $600-$700 probably without too much trouble. The CPU is fast and the graphics are pretty good also.
I wouldn't think $550 should be hard to get if it has a bit of wear but still in good condition. Best to probably list it on Ebay at around $300 and see where it goes. A couple guys get in a bidding war for it and it could go bigger.