Good advice by the guys above:thumbsupThis is a really common question. Do a search and you'll find all the info on the mods.
M50 Manifold, Throttle Body and Remap.
'US' M3 Cams.
Doing much more than the above will start to cost serious money. Induction kits etc normally aren't worth the cash.
Check out superchips website, there's a pdf dyno graph of the 328, look how much torque drops off after 5.5k rpm - it's all because of that restrictive manifold...