Presta valve setups allow for narrower rims. All of the mid to high end mtn bike rims i've built up have been presta.
I second the presta to schrader adapter, I always carry two around. I leave them threaded right onto the tube, incase I ever need to stop for air somewhere, presta's are not something everybody has.
I believe they are called "grommets', my LBS carries them for 75 cents i think