I would have thought that it replaces the psalm. Otherwise it would deepnd on who is reading the psalm - first or second reader.
hi phoebecam - you'll just have one psalm so whether it's sung or spoken that'll be it. the only problem with having the Wedding song as a psalm is that it's not actually a psalm! most priests prefer to have an actual psalm rather than a song or a hymn so you might need to run it by him to be sure he's ok with it.
I think i saw this wedding dress in the new edition of Irish wedding diary (spring 09) HTH -