Moved by your friendly neighbourhood Styleless Moderator
Well the card you have now is still good, it will be showing limitations with the hardware intensive games. It ultimately boils down to how much you want to spend and how high the quality settings and resolution you are comfortable playing games with. If you enjoy 1024x768 with a bit of AA and AF..or even without it. You need not to upgrade.
If you do consider upgrading, the only 2 (Yes there are more variants like SE, Non-Pro etc...but I'm not including those) ATI options are high end and expensive. Those are the X800 PRO and X800 XT PE. Now if you are going to get one of these cards, you'll be severely limited by your PC and motherboard. AGP8X is the requirement for those 2 cards.
If you want more options, consider upgrading your entire system onto a PCI-Express platform in 2-3 months, when AMD64's finally get solid PCI-Express motherboards.