Paul Thurrott published another article about Windows Vista this week in which he takes a look at the 64-bit support. Here's a short snip:
For the most part, the x64 support in each edition is identical. One exception is that they support different amounts of RAM. Vista Home Basic (and Home Basic N) with support up to 8 GB of RAM, compared to 4 GB for all 32-bit versions of Vista. Home Premium, meanwhile, will support 16 GB. And Business (and Business N), Enterprise, and Ultimate will all support 128 GB or more of memory. (The "or more" bit refers to the fact that there are no client PCs available yet for over 128 GB of RAM; when that happens, these Vista versions will support it.)..