from what i can see right now it looks like ati has the best card with the
ATI Radeon HD 5970
Nvidia's best card right now is the
GeForce GTX 295
on the topic of this thread if you still havent got a card you could look at one of these PNY GTS 250 512MB PCI-Express Video Card its equivalent to the GeForce 9800GTX+ or GTS+ cant remember at the moment but was the top of the line 9800 and its 99.00 at Fry's might find it cheaper online too. should be able to play cod6 at fairly high resolution with all the settings up high except maybe AA.
seems they took out the link i had posted for the GTS 250 but you should be able to find it online easy