I have a 386 with 4mb of RAM and 256k of VRAM! It's in the closet though, and I've had older hardware.
Anyway, I'd emulate Vista, but I get the feeling that Vista will be unusable on the fastest computer in emulation.
I don't even want Vista- it will be full of candy interface and security issues. I'm looking toward Windows 2007 or 2008, aka Windows Longhorn, or Windows Vista Server. If Windows 2000 and 2003 is an indication, the server version will come with things like Remote Registry, Messenger, etc disabled by default. As well, by the time Vista is more popular, moving to Server won't be as difficult. The Vista codebase will have its own awful peculiarities, but many of the rewritten things and additional features might become requirements soon after release as it happened with XP (WinME was dumped fast).