It depends on how old your wife is. 🙂 I have fond memories of a lot of '80s songs because that's when I was dating, but then again, there have been quite a few nice, romantic songs I've heard recently. Some of my favorites:
You're the Inspiration - Chicago
Heaven - Bryan Adams
Follow You, Follow Me - Genesis
I'll Make Love to You - Boyz 2 Men?
Because You Loved Me - Celine Dion
Valentine - Jim Brickman with Martina McBride (good one, TomCsTulip)
The Glory of Love - Peter Cetera
One More Night - Phil Collins
Endless Love - Lionel Ritchie and Diana Ross (or you could go for the more recent version with Mariah Carey)
Those are just a few that come to mind - but I like just about all of the slower songs by Phil Collins/Genesis and Peter Cetera/Chicago (I'm 33, if that helps place my taste).