I've got an E46 that's supposed to have around 330 BHP. The engine is naturally aspirated, and the rest of the car has no modifications at all.

How would I go around bringing it up to 400 BHP without decreasing engine life?

sorry monster you do not put superchargers on M cars and that would decrease engine life..... what you would need to do is put all the mods you can get for it just to start off these are all quick guess air intake 8 to 12 HP headers and free flow cat and exhaust 20 HP computer programmer 10 HP aluminum under drive pulleys 8 HP aluminum flywheel 10HP aluminum racing drive shaft 20 HP all that would cost $15000 or more if you want to find out for sure go to wwwbmwautosport.com or www.bmpdesign.com

actually E46's are around 240bhp....

engine life would not matter because all BMW's are autobahn engines that can sustain high speeds foreva!!!

new exhaust system. headers, muffler, cat converter
cold air intake
supercharger
new on board computer.
that should put you well above 400