Answer
This question was answered on May 9, 2006.
Much of the information contained herein may have changed since posting.
The current released minimum requirements for vista to take advantage of all of Vista's graphical schemes are as follows:
* DirectX 9 framework
* Windows Display Driver Mode
* 32 bits per pixel depth with 64MB of graphics RAM
* 2.4GHz processor
* 512MB RAM
* Graphics card capable of handling Windows Presentation Foundation
Sticking to just the classic theme, at least as of right now according to microsoft, will allow you to run it with the "classic" windows theme.