Most of the people on here have US market cars, and we dont get the 320i or 320d, you are better off with a european messageboard where there are going to be more users with 4-cylinder cars. I know there are quite a few bits out there as my friend that lives in Enland has a 320 saloon that gets up and goes pretty damn well with just a few mods (like exhaust, intake, and chip).
here is an example of other messagebords:
http://www.bmwfanatics.co.za/showthread.php?tid=10714