Well, the maximum is like 64gb or something...it's a daft amount that we won't get to before the next Windows release
EDIT: OK, Just looked it up...it seems the maximum varies from version to version...
In order, Home Basic, Home Premium, Business, Enterprise and Ultimate
Maximum RAM (64-bit version) 8 GB 16 GB 128+ GB 128+ GB 128+ GB
You're Right, the 32-bit versions all have a limit of 4gb...But if 64 bit is available you should use it..
Sorry we're going a bit off topic here lol