I don’t think anyone else here has a similar car, but please search and read the rest of the interwebzzz. You already are on E90 forum it seems, I did a quick search on Google and pulled up the below in a few seconds…
Technical Info
Conversion
http://www.e90post.com/forums/showthread.php?t=27727
Other isht
This post in particular seems to offer a good explanation of why there is a power difference in “just” the intake itself (3 stages in the 330i and 1 in the 325i) and ECU are worth that much HP.
Oh and another good read on what you are trying to do, featuring TMS equipment:
http://www.eurotuner.com/techarticles/eurp_0705_bmw_325i/index.html
Sit down though before look at the pricing. haha.
I would back check anything here with more searching- but hopefully this gets you started. You can see in one post that the intake + ECU was being offered for around $600, but who knows what they go for now.