I have a Vista laptop and the second-hand XP desktop, and there definitely IS a difference. The Vista experience was what convinced me to go with a used XP box when I needed a “new” computer. And I don’t think getting the XP Pro “downgrade” for a Vista-loaded computer is a good idea (not techie enough to have a clue why, but I’ve heard bad things — not the same as real, original XP).
I’m on a Vista machine right now and have a XP box also ... the only reason W2K can’t do everything a XP or Vista box can do is because MSFT doesn’t want to support the older OS’s although they certainly could come out with a SP5 that has all the “under the covers” upgrades and eliminates the BS limits on the level of MediaPlayer and IE supported,, they want to sell Vista/W7 ..