Jimm 12/31/2011There are no magic parts to place in the engine to gain power. Try to first have the engine tested for compression and general overall health (stae of tune). Next, examine and improve the exhaust and intake systems for less restriction - better flowing exhaust system and less restrictive intakes will cost around $1000.00 - on up. Talk with other BMW owners to see what they have used and might recommend.
Realize there is no 'free lunch' - what you pay for in terms of additional power you may lose in fuel mileage or engine longetivity.
don 12/31/2011up grade your thottle body...............