Word to the wise. If you've just got the bug now, it's never going to go away since you will never be done or ever be completely satisfied for very long. Turn away now if you know whats best for you! LOL
As far as intakes, at this point, there is only one manufacture that makes an intake expressly for the 2nd gen matrix, and that K&N. Although there may be other manufactures that may make an intake for the 2.4L 2AZ-FE engine also used in other vehicles. In fact, I have just recently ordered the Injen cold air intake that is made for the corollas, but appears to have similar placement in the engine compartment. I won't know until I get it in.
If you have the 1.8L engine check this
thread
Another mod, besides intake and exhaust, that will improve performance and fuel efficiency is to replace the crank pulley with
lighter weight or
underdrive pulleys. I'll probably do that sometime.
I visit matrixowners.com more often since there is more traffic there for the matrix