As you are using this forum I guess you have some PC building ability - why not build it yourself? you could do so for between 400-450 easily and have ths satisfaction of having done it yourself. I have just completed building a P4 system from scratch with very similar spec to yours and with XP Pro and SP2 and a good Graphics card for games etc. The grand total was £405 plus of course my time to build it, I enjoyed the challenge and have a cracking PC. Think about it.
I have ZERO PC building ability. I wouldn't have clue as to how to install a motherboard nor would I have the time.
I can appreciate the satisfaction factor that doing myself would provide but that has to weighed up against the fact that I need the thing now, I need on-site warranty and I need to finish about three songs before the end of April.