The OEM is cheap as chips.
Retail is stupidly high.
Most people will get it through a new PC purchase.
The family pack [knew they were doing it, but didn't realise you had to buy Ultimate] is pathetic.
Personally I would buy OEM, [if I were to buy it] as it's cheaper. You don't get the support from MS like retail but they'll be a gazillion posts, topics, MSKB's on all vista related problems.