As a graphic designer I use a fairly powerful PC in work (something like 2.8GHz and 512Mb of Ram) and my Minimac at 1.66 Intel Duo with 512Mb Ram eats it alive.
I assume you're comparing the same processor family here, and not comparing a NetBurst architecture P4 2.8GHz to a Core Duo architecture 1.66GHZ.
Apples and oranges.
I'll never buy a Mac until I can easily swap out components, or build my own, a la PC.
As I've said in other threads, I've been running Vista for months and I really like it. No crashes, no compatibility issues.