I 'borrowed' this somewhere ...
The 32-bit editions of Windows Vista Home Basic, Home Premium, Business, Enterprise and Ultimate, all support a maximum of 4 GB of RAM. The real variations come when we start looking at the 64-bit versions. On a system running x64 Vista Home Basic, you can add as much as 8 GB of RAM. x64 Vista Home Premium supports as far as 16 GB of RAM.
But it will take no less than 128 GB of RAM in order to satiate x64 Vista Business, Enterprise and Ultimate. 128 GB of RAM is the maximum supported physical limit in the case of these three operating systems.
However, with Windows Vista, there is a clear distinction between the maximum supported physical memory and the virtual addresses that the operating system will use. The x86 editions of Vista will not deliver full support for all the RAM installed in the case that this amounts to 4GB. And there are also some limitations involving x64 platforms.