To the OPs original point:
a. How far the wall will be from the house will depend on the depth of the lot and the particular model of home you're considering. I've seen some homes that don't have much back yard at all while others do.
b. The wall will help block some of the road noise. But you'd have to listen for yourself to see what you think, particularly if the road on the other side of the wall is one of the more well traveled thoroughfares.
Bill
