put in as much RAM as MB will hold (Might only be 4 gigs). If it takes 8 gigs, get a 64 bit OS probably Windows 7 at this point.
Look to see you can put in the MB for CPU upgrades, if a dual core or plus 1000 or 1.0k speed for cheap go for it. Otherwise stop and save for the big upgrade.
You can 5 box with that GPU, I have an old 8800 GTS 320 in mine.