well, as for DX 11, vista has that too (with a little cheat http://www.tomshardware.com/picturestory/518-directx-11-vista.html
, it will become available to the public in a month or so)
I've had Vista & Windows 7, and I think if your starting new, Windows 7 is the way to go.
I liked Vista, don't get me wrong, but windows 7 is allot 'lighter' of an OS. It boots quicker, i think gadgets take less resources, ect.
Games are usually comparable on both OSes, not really a clear winner.
Buy why go w/ something old, when the new and (in theory) better is available?