Try looking into Cobb tuning . As we speak they are working on a chip or reprograming of the '05 X ECU. I know this for a fact, I donated my ECU for them to break the coding on. They will start some dyno testing the first on June. I can't pedict their marketing date, but be patient. These guys are some of the best in the industry. Check them out Cobbtuning.com
I posted on the header topic also. Nismo has the '05 headers out. They also have a air intake w/filter.
I personally plan to do all three of these to my Xterra racer.