Personally i'd go with the OEM if you can get it in this way (you usually have to buy at least new mobo to be legally entiteled for it).
OEM version only formats one partition (the one where you want to have Vista installed).
So, if you have one smaller system partition and one huge data partition, you can format just the small one and install Vista on it, while second large one will stay intact.
Thats how i do for ages and it's the least painful way. I'm also using PING to simplify the task of restoring disk image so it goes even faster...