First you got to figure out if you got either a PCI, AGP or PCI Express slot, and if you dont know how to figure that out (which you probably do) you can contact your Computer manufacturer. After that, start searching for cards. My recommendations are that if you have or are going to choose an AGP card, go with the ATI X800, but if your going with the PCI card then get the
ATI Radeon 9250 at TigerDirect.com, which is where i bought my computer and Im probably going to buy that exact card because it is the best PCI card i could find. If you got the newer interface, PCI Express, then you should get the ATI X850, however the X800 and the X850 are pricey so keep that in mind. And if you have both PCI and AGP slots then i would get an AGP if you want a high performance card because of the better selection and better performance. But if you want more bang for your buck and you dont really need a high end card then get a PCI card. The PCI express cards are definitely the best cards out there and PCI and PCI express are not the same thing so keep that in mind also.