Your problems sound like driver problems. I would reccomend going to
Http://www.nVidia.com and downloading the detenator 3 drivers. That should help with any video problems, but you're still going to be rendering very slowley. If you can affor the Geforce3, and afford it easily. go for it, as you'll probably have it for the next year and a half, before its starts showing its age.
However if you don't want to dish out $350, go with the Kyro which should hold you over for about the next 6 to 8 monthes. and then buy again whatever is worth $120 to $180. I wouldn't reccomend the Geforce2 ultra/GTS. and The GPU is going to start handling more and more of tranformation, lighting, and hopefully physics calculations. The Geforce2 ultra draw's the line at Hardware T&L, while the geforce3 has a whole slew of new Ways to alter the geometry/lighting rather than have the CPU do the work.
This is already being incorperated into modern games, such as Dronez and Aquanox. Plus the geforce3's Image quality is top notch. Which Is why I primaraly reccomend the Geforce3.
The Kyro is an option because of it price. A mere $130, as opposed to $350. And the Kyo has unique featurs as well, such as FSAA, at a very little FPS loss, as well as 32 bit color, at very little FPS loss. Plus the mipmaping detail of The Kyro is very nice as well, worlds better than your TNT2, worlds.
The Geforce3 Can do all of the above also, but has a pricetag nearley three times as much. so you can run games at not 60fps, but rather 140fps. The Kyro is going to be punished soon for its lack of a T&L, or directX8 compatibility, but I'm sure a driver update will assist in helping the DrictX8 problem. But you cant incorperate a T&L engine into drives.
The choice is yours. You can either Buy the Kyro, and update in 6 - 8 monthes and never experience your FPS going too low (below 15 in very high action scenes) Or you can buy the Geforce3, which will currently be superior to the Kyro, but towards the end of its life span (in about 1 and a half years) It will begin showing its age.
Also keep in mind that DX8 Features will not be fully incorperated into games for about 8 monthes, about the time of your upgrade. Personally, I would reccomend going for the Kyro, and then Buing the Geforce3 MX once it comes out. You should have top notch gaming at the lowest expense that way.