Vista ultimate is the exact same thing as viista premium with a few extra programs thrown in . ONLY get ultimate if you KNOW you need the extra business and archiving tools that come bundled .
Buying it for home use is just nuts IMO
The "system builders" or OEM versions are linked to the computer they are first installed on . In theory MS can stop you transfering the OS to another computer , but you can get away with upgrades of hardware .
Vista is about to be replaced so buy vista 64 with the tech guarantee [ which is an upgrade to windows 7 ] or just wait a few weeks for oem win 7 premium which will be about $100