5th gen preludes don't need valve adjustments unlike their counterpart VTEC engines ... don't ask why. But there is a list of items they adjust and replace - call them as ask them what they do or better yet stop by and pick up a brochure.

I have a k&n filter. I bought it because its 50,000 mile between cleanings. So if you own your car for over 100,000 it pays its back. As for noticable difference - the engine does rev easier through the higher RPMs and I haven't measured gas mileage.