4-ply white board is the most common, but more galleries are showing 8-ply these days. 8-ply is really beautiful stuff, but expensive and harder to cut.
Unbuffered is more commonly used for color (because buffered board is unnecessary for color prints), buffered for B&W.
There is lots of useful information on mounting and archival standards out there. Some leading resources online are palimpsest.stanford.edu, www.loc.gov, and wilhelm-research.com.