Nvidia 8800GT is the graphics card you want. It can be found for around $250 and youll be more then impressed by it. Maybe if you got that... then see how your system is (with no overclocking).. then you got some options from there. If it all runs sweet then dont need anything.. but if the mobo or cpu is letting it down.. one or the other might need to be changed, probably the mobo, then you can overclock your core2duo cpu and get the most out of it. Also does it say somewhere what your powersupply is, name,specs?
You could turn that HP media pc into a HP hardcore gamer pc
(overclocking is fairly simple to do)