I know that the new Collected Poems has recently arrived, and it is a massive tome. But you don't really need it. Most of what Lowell wrote after "For the Union Dead" isn't that great. And in the Selected you get a great selection from what is effectively Lowell's first book, Lord Weary's Castle--which is a pretty strong book. There is also a generous selection from Life Studies and For the Union Dead (though I've bought the book that contains both complete collections). Lowell was a pretty important poet, but really, the Collected just goes overboard. This is all you really need.