PPK: The only way to know how to improve your fps is to find your current bottleneck.
Find an area with trashy FPS, and use this: sv_cheats 1; +showbudget. This will give you a bar graph. Find the biggest bar and that's your current trouble. If it says something like "Swap Buffers," then your video card is taxed by special effects, such as water or thermal effects. If it's something like Animations, then your CPU can't give you any more speed. Static Model Rendering is common in TF2. Models are faster and easier to render than BSP, but they are less optimized and because they are better they get over-used. Badlands suffers from model glut, as does parts of Gravel Pit. Depending on the bottleneck, you may or may not be able to do much.
(-showbudget to turn the graph off.)
Is your P4 a 3.2 or is it overclocked to 3.2? Also, consider an upgrade; P4 is junk compared to a Core 2 at similar clocks.