hey bro, welcome to the forums... good to have u year..
as to your question... a 325 has i think 183 hp... and a 328 has 190... so right off the bat.. the 328 is better.. and has more torque...
as to getting it to 300 hp
your looking at the basics...
-intake
-chip
-exhaust
-cams
-pulley
-and prolly a few more things...
just to let u know a little bit about bimmers... bimmers are not a inexspensive toy.. they are exspensive to fix.. and parts for the car are also extremely exspensive.. don't exspect to get a CAI from ebay, and expect hp, like little honda civics...
to properly hook up a bimmer.. u need money and time.. hope u got some!
good luck, and welcome to the forums.. again..