Unfortunately it seems you at least need a Radeon 7500 mobility to run UT2k3 at all well (no wonder it runs decently on the 800s). I tried it on my older 700 MHz iBook with the original Radeon Mobility chipset. That wasn't even worth trying. The thing might have DDR for video RAM, but the chip is just too slow and there's not enough video memory to hold all those textures, even when I tried pushing the resolution down to 320x240 by editing the config file.

I got less than that PB's 10fps.