Are you talking about the Combat Cards?  The cards on their own do not do anything other than offer you small tid-bits of information.  If you collect a whole set (one of the three rows of five Combat Cards) you will receive a bonus 15 minutes of double XP.  If you complete all three sets you receive a bonus 45 minutes of double XP.

And what about Gears of War?

Everyone above this post has been added to the list.

Also, in case anyone was curious, I contacted Mt. Dew about the "Mega" 20-packs of Mt. Dew:

@mtn_dew When will Wal-Mart start carrying the 20-packs of Dew? Thanks!

 

@JustinSaneV2 week of Oct 28, so you should be able to bank mega xp before the game drops 11/8

http://twitter.com/#!/mtn_dew/status/126986711040409600