I think you can get $350 for that. Most of this price being the 19" LCD and the OS. The computer itself is not very good. Just for you to get an idea, the computer in my sig, excluding monitor and OS which I already had, cost me $330 after MIR brand-new and, no offense, is much better than your set up. So even if your computer was level with mine, you'd be getting around $200 for the tower. But since it's not, you get less. I think you should go for $350. Don't sell below $300 though, mainly because it comes with an LCD and an OS.
Tech Tip: You'll probably make more money off this if you sell it not non-computer-saavvy people.