That's quite an old system now so even modern rpg's might not look so hot.
Prime concern should be the graphics card, more memory will help (DDR or DDR2? - DDR's actually more expensive now) but unless the game requirements specify a certain minimum amount of memory the priority should be the video card.
ATI certainly best bang for the buck atm, the high end Nvidia cards at least are noisy and suffer from overheating, not really sure about their mid-range cards.
Here's a starting point anyway, scale up or down as budget allows
http://www.overclockers.co.uk/produc...=56&subid=1673