Our strategy up to this point seems to be working, so let's apply it to Left 4 Dead, with one run at highest details and the other with 4xAA and 16xAF applied.
First, we'll test with no AA and AF:
The results are close here. Left 4 Dead's benchmarking mode doesn't capture minimum frame rates, unfortunately. But by the looks of things it's so close that it probably doesn't matter. Now let's add AA and AF:
There is still no difference. The Left 4 Dead game engine is coded by Valve, a game developer with a solid reputation for getting great visuals to work on old hardware. You can bet that the Left 4 Dead engine is making very efficient use of texture memory to produce results like this at 2560x1600 with a 512MB graphic card, although we will admit it is based on the older Source engine.