At 1024 x 768, 32 bit color, no AA or Ansio, everything maxed and physics set to normal on a GF3 Ti200 with no problems at all.
In benchmark I get around 111 fps flyby, and 49 fps bot match.
I have a GF3 Ti200 in another system that uses regular SDRAM (256 megs Mushkin PC150) and runs a 1.2 ghz TBird oced to 1.35 ghz, and on that computer also play UT2003 demo at 1024 x 768, 32 bit color, no AA or Ansio, everything maxed and physics set to normal with no problems at all too
Benchmark is a little lower, with flyby around 97 fps and bot match around 35 fps, but still looks awesome and plays great.
I think on Billy's system a GF4 Ti4200 128 meg card should be able to play UT2003 at 1024 x 768, 32 bit color without any problems at all, and probably get frame rates faster than my DDR system with a GF3 Ti200