HUGE TF2 player here.
I was able to play this game maxed on an old 8800GS. (minus AA/AF due to the small ram buffer.) If all you care about is TF2 and dont' want to keep your card for any new games then anything around that should be fine. 9600GT, GT240, 5670/6670, etc. I'd be careful going any smaller then this as you might get into trouble later with new games you might care about.