I recently upgraded from a Tbird 1.33 to an XP 1800. There was a minor increase in 3d mark 2001 se (about 700) I have seen benchmarks which show DDR systems of my spec get over 10,000. I have already cl****ed the card to 300/600 stably. I think the XP's take more advantage of the memory bandwidth in DDR.
I found that CPU intensive games like GTA3 and flight simulators have a big performance increase whereas GPU dependant ones like Quake 3 engine games or UT 2003 don't . They run well because of the GF 4 anyway. I don't think that an XP 1800 or 1900 will take advantage of the r300 or the nv30 to its fullest extent. They're going to want a Hammer or a good Northwood with RDRAM.
So the answer is upgrade only if you are willing to blow some money on a new motherboard and pc2700 ram i would say
Athlon XP 2800+
1 gb DDR sdram
ATI Radeon 9700
120gb IBM HD
Audigy 2 OEM