your pc probably slows up because of the half-speed off-die L2 cache, improving your graphics card probably wont do much to help in this particular instance but it wont cost too much to improve it which wouldnt be a bad idea, first you should accertain whether your card sits in an agp slot or a PCI slot. then if you go to dabs.com. click here
and filter those results you will find something in your price range.
keep in mind that you cannot put every agp card in every agp slot, if you need more advice post your motherboard details as well,
happy shopping H.