everything the previous posters have said is correct as right
i would recommend nothing less than a 8600gt/HD4670 for the avg joe gamer, and i would go 4GB on the ram just because ram cheap as fk these days.
crixx asked for details, that would help tremendously, especially the mobo, if it has a pci-e x16 slot then i would definitely recommend a discrete card and moar ramz if the slots permit it