First... just my opinion, but I'd have the older two share a room (even if they're opposite sexes) until new baby is sleeping consistantly through the night... To me it just seems like an infant will cause a lot of sleepless nights for the older one sharing a room with him/her. probably even through the first year since there is such a differance at that age.
I'd decorate the baby room however you would choose if it was sharing a room with the appropriate sibling, then decorate the other one in a similar theme, but neutral greens or yellows. Then when the babyis old enough to share a room you don't ahevto redecorate, you can just throw some accents to de-neutralize the room.
So anyways to answer the question you asked rather than the unsolicited advice I'm giving.

I'd use polkadots, stripes or plaids... I am particularly partial to plaid/ gingham. I don't think you can really go wrong in this area. I mean decorate the room for the elder child, by the time the infant is old enough to like/dislike it, they'll be old enough to be into the same things.