You'd probably be better off parting the system out and selling to components individually. All together, I wouldn't offer more than $200 for that system and ONLY if it comes with the original Windows 7 Ultimate DVD. The system, as it is, might make an ok HTPC (without storage).
Im going to lean to Wolfshadw's side here. Parting the system out would be your best bet. There would be a very small market willing to buy an outdated system at any cost. The video card, CPU cooler, and the operating system (Install DVD that is) are most likely the only parts worthy of resale. To answer your question $400 is too much for the system, as much as we'd all like to be able to pawn off our outdated rigs.