I've recently been searching for performance in UT2k3 as well, for reference my system:
2.0A P4 512 ECC RDRAM
Creative Audigy Gamer
GeForce 4 Ti4400
Running RedHat 8.0 (2.4.19 cust)
and the infamous x4191 drivers
The short story for those who want better fps:
Disable High quality sound, or all sound, and check your fps. Running low quality sound I average ~45 fps everywhere at 1600x1200 max quality settings. Yes, it still spikes low ~20 in areas, the engine doesn't seem to be perfect in handling the map designs given.
How do you disable high quality sound?
Edit your engine config file at: ~/.ut2003/System/UT2003.ini
under the heading...
you may also wish to adjust settings here.
I was running extremely low fps ~15 and lower with typically low fps areas running up ~45. I found that this behavior was independent of my graphics settings entirely (640x480 low = same fps as 1600x1200 maxed). That means its not graphics hardware related.
Performance charting cpu / memory / swapping etc shows the system is not under heavy load, ~60% cpu load with 1~2 processes waiting for cpu time (vmstat).
Now sound running low, the system keeps up ~45 fps with intense settings, and UT2k3 looks great