depends on which amps and subs you'll be using
You are going to want to have a capacitor no matter what you do. I have a 140 amp alternator and also a 5 farad hybrid cap from Stinger, I can still see the headlights dim a tiny bit when its really pumping...thats two MB Quart amps, two sets of seperates, and an Alpine type X 12" sub...just for reference purposes. I would get a good capacitor, at least two farad, and just check out the amperage draw once you get everything going.
<img src='http://www.bimmerwerkz.com/forum/uploads/post-19085-1125500838.jpg' border='0' alt='user posted image' />
98 328i ECIS CAI / Tint 35% / 17" Gun-Metal Gray M Countour style wheels with Polished chrome lip and 235/40/17 Toyo Proxe T1-R's / Silverstar bulbs - head and fog
System - Calrion ProAudio HU / MB Quart RA 1000 Sub amp, RA 4200 4 ch amp / MB Quart PCE 213 5.25" seperates front / JL Audio VR 6.5" seperates in rear / Stinger SPDH5 Pro-Hybrid 5 farad Capacitor / Alpine Type X 12" DVC Sub in a slot ported box