I have read a few reviews in the net and most of them could show that XP is indeed faster than Vista and faster than preview versions of Win 7. Also most games run faster on XP than on Vista. Often they even run faster on DX 9 than on DX 10.

But nevertheless I installed Vista this time to use a 64-bit installation of my modeling and rendering application (Lightwave 3d). So this software is able to address more memory for rendering. This can be useful especially when I use displacement mapping as a render option.
And I can run XP parallel with enough memory. So no hard disc swapping is needed at any time.

But a casual user really does not need such an operating system.