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