The engine of UT is not exactly one of the freshest anymore and it is well known to be limited by the processor rather than the graphics card. Athlon used to beat Pentium 4 in this benchmark and this fact still remains with the new 12.10 driver as well. However, while Athlon shows a tiny performance drop, Pentium 4 has a little performance gain.
MDK2 at 1024x768x32 is fully limited by the video memory bandwidth, so that new drivers are not able to show any performance difference. Everything stays exactly the same.