first of all i suggest you rent the games or download the demos and vids of the games on LIVE to help you decide before you take anyone elses opinion to heart.
also i dont know how you havent read or heard ANY raving about either CoD2 or DoA4 as there has been very little but raving about both games since their releases. as for NFS, i have heard both good and bad, if your a fan you will be happy, if not then i would look elsewhere.
DoA4: totally revamped fighting engine, anyone that says not much different then DoA3 clearly hasnt really played the game. gorgeous to look at and total fun online. yes there is online co-op in tag matches and you can even have a friend over and play co-op against others online in tag matches. have played plenty of matches online and lag really isnt an issue as some may say, only during tag matches where 4 connections interact with each other does lag really become a factor. as long as you stick in games that hover around 6-8 people the lag is non-existent, i have even played some games with up to 13 people and the lag was great it just sucks to wait that long to fight again after lose. the single player game has enough different modes to keep you occupied and the AI is challenging and rewarding if your up to it.
CoD2: one of the best FPS's i have ever played. single player is really fun and a good challenge in either hardened or veteran difficulties. AI is
amazing and there are countless "gaming moments" throughout the campaign. online is great as well, yes there is lag but no where near as bad as some would make you think (atleast my lag isnt as bad as described). when in a good match the online game is VERY fun and addicting. wish there were better options for online but the modes give a good variety of choices to keep you happy.
NFS:MW: like i said before i have read both good and bad about the game. however overall it has been very good, and has been said to be the best in the series to date. the graphics are impressive for a xbox port (i guess) and the fans have had very little to complain about other then the frame rate issues. i wouldnt touch it though
do some research and try to play before you buy, nothing worse then taking someone esles OPINION and regretting the purchase because you dont like the game.