This is the thing: It may look simple, and if you know your way around it is, but trying to choose the right PSU can be a PITA. Last time I bought one it was a 80 Plus Gold 500w which should've been more than enough, but it was a bag of crap (£60 worth of crap, mind) and I then ended up buying an £80 750w one which eventually worked. Still going, in fact, it's for my NAS.
So many connectors, and trying to make sure it fitted, and getting it in correctly... Ball ache.
EDIT: Still, doesn't matter now as it's fixed, nice one!