TheNamlessGuy said:
If it can display everything it would be the processor.
It can't though. The framerate is choppy.
Ping is irrelevant. That's to do with your internet.
It's really difficult to say, as all you've said is that you have a low framerate. Here's a test. Try turning everything to low, except physics, which you should set to high. If you're still lagging, then it's probably the processor, as many games offload that data from the GPU onto the CPU. TF2 IS low on textures and geometry though, so it shouldn't be very taxing for your GPU.
Better yet, do this:
If you are using XP
Go into the start menu, click on run, type in "dxdiag". Post CPU and GPU here.
If using Vista
Go into the start menu, type in "dxdiag". Post CPU and GPU here.
Basically, we need your computer's components listed to tell.