I've just upgraded my 2005 e90 330i with the following;
Underseat : Audio Systems AX 08 BMW (Direct Replacement - They also do a 2ohm 'Plus' version)
Rear : Infinity Kappa 42.9i
Centre : Vibe Slick 4
Fronts : Rainbow Dreamline C4.2
All very easy to fit and you can get the whole lot for around £400, and after some (or maybe a lot, MS-8 it an interesting challenge) of tuning these sound pretty good together
I have them driven by an MS-8 and JL XD400/4 (Fronts/Underseats).