I finally got around to doing some performance work on my trusty bimmer. The best bang for my buck was replacing the factory air intake. I replaced the factory air box with a K&N high-flow filter and a Signature Motoring heat shield box. If you use the K&N without the heat shield box, you will actually lose power in city driving due to the hot air intake. The K&N was about $50 for Bavauto.com and the heat shield was about $100 from signaturemotoring.com