From what I have read:
The N20 4cyclinder Turbo will be the standard motor. I have test driven the new 328i and this motor hummmms very nicely. Was up to 70MPH in no time, didn't feel torquey like a 6 cylinder though. But that is expected I guess.
There will be an updated inline 6 Turbo motor for the supposed 435i or 440i or 445i or whatever BMW is going to call it.
I was thinking this maybe my next car but from what I have read this will be an up market car. I am certain it will be out of my $$$ range.