I have exactly the same mainboard with GF4 Ti 4200, it's dissassembled now, but now I have active dual Tualatin with the same Via chipset and did some test with GF4, Radeon 9700 and Radeon HD4650. Not exactly the same cards you are asking about, but maybe that will help in some way. HD4650 allowed me to play Half-Life 2 in 1080p with 30+ FPS and AA, R9700 was not soo good but also worked better than GF4. For most games suitable for P3 GF4 should be fine, I played GTA3, GTA VC on it, but if you want to extremely push P3 platform and run more modern DX9 games, high-end ATI card may help.
Here's a video of HL:2 with HD4650, unfortunately I don't have one recorded with GF4. It worked, but had worse performance. https://youtu.be/MlbITJMZeJM?t=174

I'm not terrible sure, i compared my 9800 Pro on my XP 2800 vs my AThlon 64 3200 and it jumped 15%, at P3 level of preformance it might only preform like a 9600 Pro would in all honesty

Yeah, I think you should be able to push the graphics settings and resolution higher, but you wouldn't gain much in terms of frame rate because the Pentium 3 would bottleneck your gaming performance.

Aha... Looks like it is worth a try...
Thx for the replies ... I will test it with the gf4 and then with the 9800xt -

at the moment I am awaiting the delivery of a silencer ATI 3 ... The cooler of the 9800 xt is loud as hell.. that's no fun