I just go today the K&N 77 Series High-Flow Performance Kit. I will install tomorrow, and let everyone now how it went, and the differences I can feel and hear. It was $240 from eBay.

having installed one on my x, i have a small suggestion. #2 on the instructions, i would consider changing the original screws on the mass air sensor and using allen wrench bolts. Doing some minor maitenance I actually stripped the originals and thought it would be better to change those 2 screws. I also used lock tight on all the bolts for the heat shield. be carefull with the mass sensor, its about 400.00+ to replace. I disconected mine and placed a very clean rag around it to keep from getting damaged if it would fall into engine compartment. good luck