UT2K3 is a new game, and new, graphics-intensive games like this usually tax current systems quite a bit. The best option, though usually the most expensive and least possible, is to upgrade, either your processor, RAM and video card (which you just got), or a new computer. However, barring that, you can try tweaking your system to use less CPU cycles and RAM, and tweaking your game to be less demanding.
Test your system with Quake 3, as this is the de-facto for comparing fps's (frames per second) on FPS's (first-person shooters). You should get excellent results, as you did with most of the other games you tried.