I would recommend Nunnally over Archer, and you can find it in one volume on Amazon (and possibly at your local bookstore). It's suppose to be more in the style of Undset's original prose. The Archer translation uses a lot of archaic language (think words such a trow, liefer, etc.) which makes it more awkward to read until you find your rhythm with it. Another compelling reason comes from Nunnally herself in an interview I read:
For some reason, the first translators (Charles Archer and J. S. Scott) decided that since the book was set in the early fourteenth century, the language should sound medieval. They created an artificially archaic style of stilted dialogue and convoluted sentences, which they imposed on the author's wonderfully clear prose. The old translation makes the novel read like a bad imitation of Sir Walter Scott-full of such words as fis, twos, thee, thou, methinks, hath, and doth. There are also numerous deletions and some rather serious omissions that clearly fall under the label of censorship.
Hope this helps!