I know your feel, I run on an average of 20-40 fps and i get 5-10 fps on specific maps. However, on Jump/Surf maps i get around 150-200 (Since those maps are designed completely different than regular maps)
I suggest running at these settings:
-Chris' Maxframes FPS Config
-640x480 Resolution Fullscreen
-Launch commands: -applaunch 10 -gl -nojoy -noipx -full -noforcemaccel -noforcemparms -noforcemspd -novid -nocrashdialog -nocdaudo +mat_forcehardwaresync 0 -console -32bit -w 640 -h 480 -dxlevel 80
(Some of these may or may not work anymore)
My pc specs so you can get comparable results:
CPU: AMD Athlon(tm) II X2 250 Processor (2 CPUs), ~3.0GHz
RAM: 4096MB
GPU: NVIDIA GeForce 6150SE nForce 430
Monitor: Generic PnP Monitor 1360 x768 (32bit) (60Hz)
So yes, TF2 is totally playable. Will you get the best results? Probably not, I also suggest you to cap your FPS if it starts to get jumpy (Going from say 120 all the way down to 30 at random points) So your fps is consistent. Also, i suggest you to check out this thread as it has some other ways to boost your FPS that was not listed here:
http://teamfortress.tv/forum/thread/7639-fps-improvement-compilation
Good luck :)