Then go for 6*9's in the rear, here, read my thread on 328i.org on the rear speakers replacement
http://www.328i.org/viewtopic.php?t=120
I went from th stock rear speakers to pioneer 350watts 6*9 and I love them, I have had those for a while, so when my rear speakers busted, I just put those in so I wouldn't need to buy new ones