Sorry too lazy to type it all again, here's a thread you can read more about it:
http://www.bmwboard.com/forum/showthread.php?t=15892&highlight=supercharger
I would suggest registering at
www.bimmerforums.com and joining their FI forum - many, many folks there that have Supercharged and Turbo'd all sorts of BMW's (Execessively on 3's of course). From there you can get all the good FI exclusive site links (I'm on my work PC or else I'd have a list for you - I'm shopping too)
SC's are fairly safe these days to apply to most cars. Moreso than turbo kits, where you usually have to strengthen the engines internal parts to run a Turbo kit of any decent sort of PSI level. Your model car can help determine this (BMW wise at least). Models with cast parts, I wouldn't even think of putting a turbo in - cars with forged parts (stronger) could take a small turbo without having to upgrade too make stock internals. (Just a quick overview there of the 2)
For a simple Stage 1 SC on your 325, you're looking at about 6-7k for the SC, and about 1k in labor for install. You'll get about 90-100 hp, and about 60-70 more torque, putting you in E36 M3/G35 range easily.