ReadyBoost is not RAM. It's somewhat analogous to having a pagefile on an SSD.
Hence, the "bitness" of the OS won't have any direct correlation on the effectiveness of ReadyBoost supplementation, and I wouldn't recommend going to 64-bit purely for that reason, unless somebody's doing it to experiment in which case good on them
With RAM prices as low as they are currently, throwing another couple o' gigs in there will improve performance much more than readyboost.