Apple really screwed the pooch on this one. I own an original Mac Pro -- it's been a fabulous machine and is still screaming fast three years later, but it will only recognize two gigs of RAM running 32-bit Vista. This isn't a Windows thing, mind you, it's a limitation in the Mac firmware.
So, this morning, after doubling the RAM in my machine, I decided to upgrade to 64-bit Vista. The problem is, Apple doesn't support it -- see here.
Not only do they cripple 32-bit RAM support in the firmware, but they don't support 64-bit installation on my machine. It doesn't sound like interoperability is really a goal in Cupertino.
The Apple support forums weren't helpful, so after searching, I found a solution on MSDN of all places. The thread deals with installing HyperV, but it works for Vista as well.
All you have to do is boot into Windows, copy the contents of the Vista install DVD to a temporary location, and use oscdimg.exe from the Windows Automated Installation Kit.