
Powercolor ATI 5850 1GB GDDR5 PCI-Express - $50
It will run NS2 at 1600x900 with high textures (I can't remember if I had AA/AF on) at over 60fps (provided your CPU is fast enough - i.e. 4Ghz or faster)... and at that point, it will be your CPU holding you back. If you try and run 1080p (1920x1080), this card will be the thing holding you back).
It's able to maintain full 1080p in COD: Black Ops 2 at constant 60fps with vsync on, but will run over 120fps with it off, but the frames rates will be more eratic (up and down between 100 and 150) and I notice a lot more screen tearing (which should not be noticeable if you happen if you have a nifty 120Hz monitor).
TF2 runs super high FPS -- it's not even an issue.