i guess there are really 2 main ways you can do a cold air intake. you have the ultra cheap pvc piping cai's which are just an extension from the air box (which personally i wouldnt put anything of the sort in my car) and you have the metal equivelent.
the pvc version will give you mid ranged gains, the flow will not be terribly great as there points where the pvc connects with another piece of pvc will cause turbulence of the air flow, you are also limited as to the bends you can have. 30 degrees, 45 degrees and 90 degrees. the 2 big advantages of this setup is cost (under $30 of piping) and you get cold air.
the metal version will give you the highest gains possible provided it is made right. what i mean by being made right is if there is minimal or no dags internal to the piping, the bends are nice smooth mandrel bends, and there are a minimal number of bends in the piping. avoid sharp bends where ever possible. you can then, if you like, heat wrap the piping to allow your engine to perform at its absolute optimum. the advantages are obvious, high air flow, low turbulence and cold air. the problem is of course the price. you can expect to pay anything from $200-400 for a good cold air intake and then around $80 worth of heat wrap to cover it. it is a lot of money, but it is defiantely worth it. you dont just hear the induction noise, you really feel the gains too.
there are threads in the faq section on both the pvc and metal cold air intakes for the u12, the information can be directly applied to the u13 without any problems.