There are 2 cats per bank, an upper main and a lower post-cat per side on all 2001+ Xterras and CA/MD/ME and a few other New England state-sold 2000 Xterras. The rest of the 2000 Xterras only have the upper main cats.
I don't want a header if it's not emissions-legal. If anyone is to make an emissions-legal header for an Xterra/Frontier they will have their work cut out for them. No doubt it will have to provide for an 02 sensor per bank at the collector and still leave room before the turn to horizontal for a suitable aftermarket cat. Dealing with the lower post-cats might be as simple as taking them out and installing some inexpensive doo-dads that mimic a "good" signal from what would be the second set of 02 sensors (just heard about those recently) as long as there is no visual inspection involved.

Here in MD for the emissions test on 96+ vehicles they just plug into the OBD-II port. No codes, no problem, off you go. If that can be satisfied without any trickery that truly defeats the purpose of the system, I'd consider it.

Brent