The only thing I can think of is the fact that your videocard is downright horrid. I can't say it any other way, but it is. The old GeForce 2 Ultras are more powerful than the GeForce FX 5200.
Even with that card, UT2k3 should run at least sorta decent. If it doesn't fix, you might as well buy a new videocard anyway -- it's bottlenecking your lovely P4 very badly.
-Lasereth