Left 4 Dead 2 (60 FPS):

Aspect Ratio -> Widescreen 16:10

Resolution -> 1280x800

Display Mode -> Full screen

Anti-aliasing Mode -> 4x MSAA

Filtering Mode -> Anisotropic 16X

Wait for Vertical Sync -> Enabled (Triple Buffered) or Disabled [If SteamOS frame limit is used]

Shader Detail -> Very High

Effect Detail -> High

Model / Texture Detail -> High

Multicore Rendering -> Enabled

Paged Pool Memory Available -> High