IT depends on whether or not you plan on enabling some of the extra fluff in Ultimate or not. Otherwise, at the base level, they're exactly the same OS. Just like XP Home and Pro. The only real difference lies in what extras are bundled with. In the case of Vista Ultimate, you have a bunch of extra resource sucking options like animated wallpapers -- which gets old, real fast, trust me... Linux has had this feature for over a decade, and virtually everyone turns it off after playing with it for 5-10 minutes -- and that's about all that sets it apart from Home Premium.
Just wondering planning on building a computer and there is a $13 dollar price difference between them on newegg.