I'd hold on to your GF3TI200, if you had a top CPU then it would be worth considering but the GF4TI cards won't produce much extra because of the CPU. Save your cash and see what ATI and nVidia do next would be my advice. Also the GF4TI4200 would be a wiser choice (unless you have money to spare), it is faster than GF3TI500 and Radeon8500, won't devalue as quickly as the 4400/4600 cards, o/c fantasticly AND it costs a lot less too. GF4TI4200 $140, GF4TI4600 $300.
Anyway, here's a few 3Dmark2001 benchmarks to give you an indication of what you would find with your current Athlon1.2ghz
(gfx card: total marks, high detail games FPS):
1024x768x32:
GF3 TI200: 6500, 37, 54, 48, 35
GF4TI4200: 7500, 35, 81, 47, 35
GF4TI4600: 8500, 37, 92, 48, 56
1024x768x32xAA:
GF3 TI200: 4400, 64, 59, 65, 20
GF4TI4200: 6000, 30, 66, 39, 25
GF4TI4600: 6600, 30, 76, 41, 34