txwood, dust collectors are sort of like catching fish, bigger is better. Buy the biggest you can afford and don't look back.
If using for metal dust, consider the heat of ground particles. My intake accumulates bonded, vitrified metal on a 1/2" mesh screen located 16" and again in a 90 degree elbow 27" below the grinding point which has to be broken/chipped off.
Metal ducts/flex/fittings are a favorite. Keep flex ducts to a minimum. Using heating vent pipe/fittings is cheaper than buying the special stuff sold for collectors. Ground the ductwork, your crotch will appreciate this very much, believe me!
Bruce is right on with putting it outside to keep it quiet. Some of them list the decible noise levels and offer a muffler for the exhaust. I think the two stage units are best, mine dumps into a 55 gal drum first, then exhausts outside into another container, avoiding the cloth bag due to fire risk.
Even with the dust extractor, I wear a respirator and find the particle filters get dirty after a while.