I agree with the other posters - GeForce2 GTS 32mb is the best card for your current system.
GeForce3 is the best out there, but realistically not a lot of software is developed yet to take advantage of its improvements over the GeForce2.
I also heartily agree that building your own system is the way to go. I've been building my own since about 1993. It is very therapeutic, it makes you more familiar with your system, and you can be assured that the final product has the best components. It provides for a much easier path in upgrading individual components as time and technology allow. You can also often find people less involved in the "lastest and best" to cheaply sell your own components to, which cuts down on upgrade costs. Many of the Musketeers have built their own systems in the last four months - most of them had never done so before and were skeptical, but the more experienced members of our squad coached them through the process. They are all extremely happy with their systems.
Citabria, if you need any help or advice, please email me and I would be happy to oblige.
Nethawk