8.4 64 bits is a hybrid release, where as the java stack is 64 bits, but the bibusses are 32 bits (thus run in wow64).
All native client drivers that you need on the server, need to be 32 bits to be talked to by the bibusses.
It only helps to get a cheap way to allocate 16 gig mem, as 64 bits OS standard addition can do that.
(32 bits would require expensive enterprise OS to go over 4 gig).
You would need a 64 bits java stack as from about 1700 concurrent users (for the CM service).
So if you have small server(s), stick with 32 bits all the way.
If big boys, get a cheap 64 bits OS, and then it's irrelevant if you install 32 or 64 bits cognos on it.