your kind of between a rock and a hard place, to get any good performing video card, you will need a new system. look into the 5200 and the 9200, they are the 2 most powerful pci cards i can think of. it will be better then what you have, but not quite as much as i think you want for gamming,
what games were you planning on playing?
http://www.newegg.com/app/ViewProduc...102-434&depa=1
http://www.newegg.com/app/ViewProduc...145-087&depa=1