If you're willing to do a capella and looking for a more solemn song, Set Me As A Seal by Rene Clausen is really, really pretty.
We did it as a quartet at my cousin's wedding, and fiance and I want it sung it ours too.
Here's a recording with a large group, but it really is equally pretty as a quartet.
http://www.youtube.com/watch?v=Qb6uY-y9HRI