AFAIK Epic states that the OpenGL rendering in UT2003 is not supported and is a lot slower than the directX rendering in the game. This is probably the reason for the low frame rates

actually, i found that i got much better framerate in linux than in windows. it could be do to the operating system; in fact, it probably is. i dont know why they didnt get better fps. may be a dumb question, but did they install the nvidia drivers or did they use the xf86 drivers for the vidcard? and were they running that massive sludge kde in the background? both of those things could slow it down considerably.

That is assuming that there are native nvidia drivers for the AMD-64 architecture yet, i`m not sure if there are or not...
Secondly, AMD-64 is a new and not so widespread architecture, and as such the compilers wont be so mature as those for x86, so there will be less thorough optimization. Compiler optimizations will take time and lots of user testing to get working nicely. Remember that AMD-64 hardware is not generally available, so many of the people developing software for it are doing so under emulation environments, which will not only be slow, but may not accurately emulate the instruction timing of the finished processor.

well there are certain background services u really dont need actually in windows.