It doesnt take a lot to run, two used 8800GT's for $160 will run the game on max with over 125 fps.
Something tells me that guys who are that dedicated to the game...will throw the bucks down for a good card. Casual gamers would be less reluctant to spend the money for it. Heck im a casual gamer and i still dished out $200 to play it...and im far from rich. Ive tried playing with low settings, and it made it harder for me to get kills.
Good performance can be had now for the price of entry level cards years back.
like i said its personaly preference, some people prefer low graphics and can see more without all the things cluttering your screen up, others like eyecandy, but ive never once saw a good cod4 player using high graphics.
Actually you have it backwards. The people that are dedicated to the game and are highly competitive don't really care how the game looks, as long as they win. It's not just about FPS, it's also about vision. If you don't know what I mean then try the Yitch 2 or 3 config and you'll see.