Ok, first thing you need to do is go to this link...
http://www.unitedbimmer.com/forums/thread59.html
as far as gains go, with an intake you won't get many, at all. But you will get a SWEET sound from your engine. Any power increase will be more noticeable in the upper RPM range, and may lower your torque slightly.
The underdrive pullies however provide a better increase in power as well as torque. But you won't get the sweet sound.