Have you used Windows 7 64-bit?
I have found that Creative Suite 5 (Design Premium) runs better on Windows 7 64-bit than on 10.6.x. It is more efficient (RAM and CPU usage/utilization) and some of the more complex filters (like Photoshop's Surface Blur and Nik's Dfine) calculate more quickly in Windows. However, the hardware at this point makes those gains immaterial nowadays, but I just wanted to point out that in my experience the days of Adobe products running better on OS X are a thing of the past.
However, if you are doing any design work that will end up on the web, you need Windows. Whatever you think of it, you need a way to see what your work will look like in Internet Explorer;
BECAUSE.
I still prefer Mac OS X and sold my CS 4 and CS 5 licenses for Windows. I wish CS 5.x had better support for Spaces (still buggy), but my workflow is better in Mac OS X versus Windows 7. However, whenever I do something for the web I fire up Fusion and take a look at what it looks like in Internet Explorer.