What length travel do you plan on running?
They appear to only come in 8"+ of travel.
That would require you to run a shock hoop or extended shock tower.
Stock won't cut it.
As for valving, check with eshocks.com, but for an SUV based on their website, you should go with 360/80.
Or if you're running two of them per side, then 180/75 for each.
http://eshocks.com/bil_Orgd.asp ETA: When I spoke with eshocks about possibly running the bilstein 7100 series coilovers without torsion bars, they gave me bilstein's direct phone number and I spoke with the racing department. They advised against running the 7100 series coilovers due to the 14mm shaft thickness. They said that an Xterra would probably just bend the shafts.
Just something to think about.
Race runners have a 7/8" shaft thickness, threaded bodies and can be run with or without coil springs.
So at least you have room for future improvement if you want.