I know how that upgrade thing goes. If you still want to upgrade your computer on a budged, go entirely used. Look for older PSB, Axiom or similar speakers used locally or on Audiogone. I picked up a refurb Sherwood 5.1 receiver for $75 CDN. I missed out on a used Yamaha high end receiver for $40 a few days before that. You should be able to keep it to less than $400 and get yourself really good sound on the computer.
Another option is to pick up a pair of Behringer B2030P active monitors. They are about $150 a pair in the US. The measure VERY flat on and off axis and have built in amps. They did very well in a recent Audioholics bookself speaker shootout.
