If they are good quality fans then you may not notice the difference, but in general larger is more quiet.
A 750 PSU should deal with anything less than XFire.
I am assuming cost is not a big issue so I guess it doesn't matter, but what are you doing to need 8-12 gig of RAM?
A lot of formula development and calculations while i watch videos, play games, etc.
Been blessed with some unexpected cash and choose to use my portion on a new rig.