I don't have the full release of Vista yet but I've run the beta versions and RC1 and RC2 through signing up for the beta testing. I quite like it although it's a little bloated and takes up too many resources for my taste. It's easily streamlined with a little tinkering though. DirectX 10 being Vista only is because it's fundamentally different to DX9 and not just an update to it and XP can't run it.
I had a mac G5 that I bought for design and 3D work but it was actually slower than my PC of a similar spec. When I bought 3DS Max, which is PC only I decided to sell the G5. Truth is for web design, 3D animation and modelling software designers put more resources into PC development than Mac. Macs are pretty and the OS gives you less trouble than Windows but ultimately Windows has so many resources to call on in terms of software, vendor support and sheer horsepower if your looking at a workstation with something like a Firegl or Quadrofx.
Hold on though can't you have the best of both worlds cos doesn't the new OS X running on a Core 2 duo dual boot between both operating systems with a little tweaking.