Not much out there for a huge addition in HP gains for the 325i. The only one i know of is Turner Motorsports. But it costs a grip and will most certainly void the warranty. Also, I don't know anyone here who's done this yet either. Have a look.
http://www.turnermotorsport.com/html...CT_ID=TN52-ST3
Unfortunately there aren't any turbo kits for the N/A cars...at least not yet. It'll be a few years until they develop it since most are working on 335i upgrades.
Your best bet, sell your car and get an M3 or 335i.