Ah yes, it's the old, "I only want to build this wall once and want it to last forever!" attitude. Like most anything, lumber is available in an almost infinite number of species, sizes, grades and finishes. This isn't because the man at the lumber yard is trying to confuse you. Using the proper material can literally either "make or break" a project. Douglas fir, for example is an excellent product for residential framing. When green it's easy to work with, prone not to split and dries hard as a rock with a minimum of deflection. Rough redwood or construction heart redwood have typically been used for retaining walls. This is because redwood, although not a strong product, has certain properties which resist insects and fungus growth caused by earth to wood contact. In as much as pressure treated fir is concerned it gives you the best of both worlds -- strength and longevity. Pressure treatments, in which preservative chemicals are applied under pressure to obtain maximum penetration, afford the greatest protection for lumber and plywood against decay or attack from your friendly neighborhood termite. In our opinion, non-pressure treated material should be restricted for use only for work not in contact with the ground. You may find pressure treated fir to be slightly more expensive than construction heart redwood, but the savings in the long run in not having to replace the wall will be well worth the additional expense up front.