I use Windows XP on my gaming PC and on my torrent box.
I have a Windows Vista / Debian dual boot on the PC in my room.
I have an old laptop running O/S X and an even older one running Mac O/S 7.5.

Windows XP is my O/S of choice.

And do you feel your visual quality has improved with Vista? So far, DX10 has not done much for gaming, that I see anyway.

I would rather have the frame rate.

DX10 on XP would be the perfect thing! To bad MS cares more about OS sales then supporting the people who already purchased there products. 

Oh, and by the way, I have an MSDN License. I can use any MS product for free, and other then Office and Visual Studio, it all sucks. (Well, Visio is cool).

Just wanted to state that, so you know I can have Vista Ultimate for free, and still don't use it.

Ubuntu GNU/Linux 64-bit edition

 I agree, but future games may support it more. My frame rate is fine. I don't know why alot of people hate vista besides the fact that it hogs alot of the rescources. 

I am pimpin Windows Vista. It's a POS but ya know. It's what came with it. I am thinking of installing MAC OS X Leopard.

Here are some differences in the two. There also is differences in the physics.
http://gizmodo.com/gadgets/pcs/directx-9-vs-directx-10-worth-upgrading-to-vista-for-243099.php

XP and Ubuntu, depending on my mood.(currently in XP, though, due to some wifi issues that cropped up after I upgraded to the latest beta of Hardy Heron)

XP on my gaming rig, Vista on my laptop because it came pre-installed.

plane old XP.