Do you have a COA for the OS?
What is the processor speed?
I don't know about the sound system but the rest isn't worth much (no offense), maybe $200 with a valid copy of XP pro and 2.5ghz+ processor.
You have to realize that computers drop in value very quickly (ever heard of Moore's Law?). I just built a 3.1 ghz dual core intel with 4gb of ram DVD, and 160gb sata HD for less than $300. You can buy brand new systems better than what you have with a 17" LCD and warranty for around $400.
I am not trying to put your rig down ( I have a couple of machines that are about the same specs), I am just trying to give you a honest estimate.