What game are you trying to play? Pricewatch.com has AGP Geforce 7300gt or Radeon 3850 that would help you a lot. You really need a better cpu though for most games now.
2.4 P4 is the minimum spec cpu for Sims3, according to Can You Run It website , so it will run. Any gpu on this link will work, just pick depending on the money you want to spend.
Just pick depending on the money you want to spend? That's pretty bad advice as there's lots of bad overpriced cards. Considering I wouldn't trust his PSU with an HD3850 there's basically 2 options; an HD4650 or HD4670. The 7300gt that you mentioned earlier is several times worse than any of these cards.