Well the 32 bit version, the max is 4 gig. That's the max size you can address with a 32 bit address. The 64 bit built removes that limit. I don't know the exact amout for the 64 bit build, but it's way more that you can actually put on any Sun box.
How big are your databases/total server size? Why are you going with the 64 bit build over the 32 bit built?