The smartest thing to do if you want a faster car, is to go buy one. That being said, if you just adore your ride and don't mind spending some money you might think about what i did.....
Had a ''95 325is, dropped in a '96 328is engine, converted the obdII system back to an OBDI System (click
HERE ), and combined with a few other bolt on mods, I have about the same performance, hp,torque, etc. of a '95 M3. This is not a cost effective project; but if your dying to yank an engine that is the most bang for the buck!