If anyone is interested in a much less expensive alternative to a K&N air filter, you can check out a Spectre air filter. I ordered one off of Amazon for $27 (free shipping) and there is a $10 mail in rebate on it. That's a lot less than a comparable K&N filter of around $75.
Here is the link to Spectre's site for the IX drop in filter that I ordered:
http://www.spectreperformance.com/#CATALOG.888208
Here is the rebate (good until June 30, 2010):
http://www.jegs.com/rebates/spectre/Spectre2010.pdf
Spectre rates all of their filters for cfm and hp. This particular filter is rated 1120cfm and 510hp before it becomes a restriction.
They also said that they have seen gains in the 2-3hp range by installing this filter which is in line with what I read where others have done dyno testing and the only thing changed was going to a K&N filter.
As far as filtering ability, on the box it says "All Speed by Spectre hpR filters are tested at independent labs using ISO 5011 standards, and have been proven to filter 99.6% of particles."