ajdisse,
I think it really depends what you want to get out of your media centre expereince. If you are after a solid workhorse, definately go XP now; if you are not a tinkerer and like to play outdoor sports, go XP now; if you have a facination with getting the latest / greatest, don't mind staying up until all hours with as yet barely seen issues, go Vista now.
Stuff like MCE will move fast in Vista, in fact it already is. There will be M$ updates that will break things you least expect, many years of M$ history tells us all to expect this.
I haven't got into the detail, but if you buy a machine now I'd be ensuring that you get a VIsta upgrade uncluded. Most major vendors will have this already. Just make sure the upgrade gives you the "right" version of Vista for you.
Personally my 2 MCE boxes are XP and will stay that way for a little while yet. They are devices used in the house. Just like I wouldn't tinker with a working fridge, I don't play with them!
Once I get familiar with Vista in '07, I will make some time and eventually upgrade both boxes systematically. MCE2 was specd with Vista in mind over a 4 - 5 year useful life for many components.
