Twin turbo Eruo Spec ///M3 engine into my 328is (I would of course keep the 328is badging and leave the rest of the car stock). Custom stainless exhaust system, stepped design from headers back (probably 2 1/4 inches to full 3 inches at the muffler). Multi layer, multi clear coat, jet black paint job (to the point where the car always looks wet). Short shifter, tranny and clutch able to match the engine.
And when I say everything else stock I mean stock side skirts, bumpers, lights (well, I'd smoke the lights), stereo (I'd only upgrade the speakers to newer ones), rims, and interior. My ultimate goal (in either case) is to have the ultimate sleeper. Mmmmm, sleeper.