Judging by your current vid card, you have a motherboard with an AGP slot. The very best card, which happens to be an Nvidia, is the 7800GS.
There is a swath of other cards that are really good (ATi and Nvidia) for AGP that really varies depending on what price range you are looking for.
When I am comparing value of vid cards, I usually check out the Interactive VGA Charts @ Tomshardware:
By looking at that and deciding on how much money you have to spend you can make a good choice on what vid card to get.
Make sure to post here when you think you have a decision on what vid card you are thinking about getting as there may be known problems with that particular vid card or incompatiblities with the rest of your system