Also, make sure you write your guest's FULL NAME on the RSVP card. That way if the bride and groom are doing place cards they have it, and if they keep a spreadsheet of guests + gifts and addresses (like I did for my friend's shower), they have the full name for thank you notes.
Although, it's been forever since I've received an invitation as "agzg and guest." Live-in's stuck around long enough that my friends all know not only his first and last name, but middle and probably social security number or something.
Facebook makes some things easier.
I should add that for weddings for his friends and family, they address it to both of us, as well.