Which graphic card is most suitable for me?

I plan to learn 3D animation software and I have purchased a workstation with specifications below:
Tyan S2462 Thunder K7 motherboard with dual AMD Athlon MP1.2GHz, 512MB of DDR RAM (buffer/registered), SCSI Harddrive & Matrox RT2500 Video Editing Card. The operating system is Window 2000 & the 3D software I would like to study is Newtek Lightwave 7.
Now I’m confuse about what type of graphic card I should use. The existing graphic card is GeForce 3. Everything looks okay but I heard from my friend that nVidia GeForce is more suitable for PC Gaming but not for 3D application. I try to get info from internet about OpenGL & Direct3D but what I understand that the latest GeForce 4-Ti or Radeon 8500 is also powerful enough for 3D rendering, T&L, etc. My questions are:
1. What is the ‘practical’ difference between nVidia GeForce 4-Ti & nVidia Quadro 4-XGL?
2. Can the GeForce 4 run under Win 2000 & match to the Lightwave 7?
3. For 3D application, which feature of graphic card is more important? the speed (MHz) or memory (DDR)?
4. If consider about balance between cost/price & result/performance, which graphic card is most suitable for me? Thanks.