Well at least the guy (EVENTUALLY!) took people's advice, ran the benchmark, and even posted the results which showed his initial claims to have been rubbish - it takes a bit of maturity to be able to say "yep, I was wrong, this is my actual framerate".
As to whether 20FPS is playable, well, an acceptable framerate is more about what the individual player is willing to put up with rather than some objective formula like "if FPS>30 THEN="playable". I definitely wouldn't want to play it at that speed, but since dvdbunny evidently has been playing it and enjoying it then we must surmise that for him it's "playable". No doubt he'd get slaughtered in multiplayer by someone shooting him in the back while he was turning around and his PC was agonizingly trying to redraw the screen but in single player it's good enough if you're willing to put up with some choppiness. I'm sure if he saw it running at 50fps the scales would fall from his eyes and he'd find going back to 20fps painful, but what he doesn't know can't hurt him!
I remember running Wolfenstein 3D on my 12MHz 286 - thinking back to how it ran I thought it was godly, but I'm sure it was far from it! Or trying to squeeze Quake on a 486 lappy, having to reboot into DOS to free up enough RAM to launch the game, and reducing the screensize to the size of my thumb, but I was still playing it cause I was 16 and frothing at the mouth at the thought of playing this fantastic new game and gawking like an idiot at how great it looked!
@dvdbunny: why don't you drop the stupid 4xAA? You'll find the increased framerate to be a much greater improvement than the marginal image quality benefits antialiasing provides.