Dinan (
www.dinancars.com) makes the exhaust, intake, software, suspention upgrades, tower braces, and throttle body that I personally use on my M3, so I highly recomend looking into them, if you can afford the rather steep price. I know for sure that they make software, suspention upgrades, a cold air intake, and a free flow exhaust for the 330/325 models for a modest 197hp and 182 lb/ft of torque (+12/10). Handling improves as well but I dont have numbers for that. Costs between $2,500 and $5,000 depending on how much you want
Its pretty hard to get extra power out of the E46s, but I've read articles about the Active Autowerke supercharger, which gets very noticable power gains. Costs around $6,000.