I know you would like to keep the price down, but to handle 5 boxing in the top tier areas, you need:
8 gigs of RAM which means 64 bit OS. Try to get a free upgrade to windows 7 when you buy Vista. If you can put it off say 2 months then Win7 will be out.
Better Graphics card A 9500 is on the weak side, are you using dual monitors? Ask what options they have.