durban is right, you generally want an i3 or i5 processor(maybe an i7 if you want to stream) to play tf2 with a higher frame rate even with dx8 settings. (think 200-300 fps)
Keep in mind if you upgrade your CPU that often means upgrading your motherboard as well.
So you can certainly try upgrading your graphics card, but that doesn't mean your CPU won't bottleneck the rest of your system especially when playing TF2 which is very processor intensive.
I'm pretty sure enigma mentioned he was running a 9800 GT a fairly old graphics card at this point but he had purchased a new i5 2500K and was averaging 200-300 fps.