Will probably dual-boot at first.
I switched from WinXP SP3 to Vista SP1 a few days ago and here is what I have found so far:
There is no difference in boot speed (maybe Vista is a tick faster).
IMHO it is a good idea to disable superfetch cause it sucks that Vista is accessing you HD endless after boot slowing down other apps meanwhile.
The whole MP GUI seems to be a bit more sluggish on Vista compared to XP.
grubi.
If setup correctly Vista SP1 actually seems faster to me. For Media Portal use just disable all effects and what not make a black background and put it to the classic start menu bar. What I'm doing for my HTPC.