I have a very capable PC--GTX 580SLI. I have that config because I appreciate high end graphics. I have played B3, Crysis 2, Metro 2033, and other such titles. Yet, my fav online game is still CoD4, and I find the graphics to be pretty decent. I actually like the gun models in that game better than those seen in many of the newer game engines, and the maps are very good too There is too much emphasis on graphics over the last few years to the exclusion of decent game play, and story telling. That said, the CoD franchise has been equally guilty of offering up lackluster short single player campaigns as of late as well.
Given the choice, I would rather have a great game (long single player campaign with good story and character development, and good online play with dedicated servers) with the present CoD engine, than a mediocre game with stellar graphics. Both at once would be the ideal, but given a good game to begin with, I do not find the current CoD engine to be a problem. I would much rather play Hl2 for a second time with it's 2005 graphics than MoH 2010 with the Frostbite engine for a second time because MoH was so short it felt like a rip off. The multi player was not very good either, whereas HL2 online was awesome.
I just bought CoDMW3 on disc--(for $20.--that's why I wait. I actually played the Beta for MW3 when it came out over a year ago). I got MW3 mainly for the single player, as online has no dedicated servers--so I will just stay with CoD4 for multi player. Hopefully the story for MW3 is better than CoDMW2 which was a let down.