*Ahem* So yeah, Aero is pretty and it works well with my BRAND NEW LAPTOP which I purchased recently..
And this is the crux of Vista to me. Although the NT kernel is, IMHO, fine, the OS on top of it all needs a lot of work. Regardless, what I'm saying is, the changes make much old hardware have issues (older LSI chips simply don't operate at the frequency or order that Vista requires, and that demands much more complex drivers for compatibility). New drivers also get more attention. I think Xp should be for any machine older than 2 before Vista (Jan 31, 2007). Anything newer is great with Vista, with support, and speed, before that, it should probably get cut. It's like running OS X 10.4 on a G3, doable, but not totally recommended. Many older iBooks with FW were supported for Tiger, but I saw many issues with the ATI graphics that were never quite fixed.
I wouldn't plan on updating any G4 Macs to Leopard either. There's that difference between supported and running well. Bottom line, Vista is best on new hardware put together with Vista support in mind.