I was just introduced to the music of the Vitamin String Quartet. How did it take me this long to find out about them?! They are incredible! They completely rock the whole I-want-cool-music-but-want-it-to-sound-classical thing. Check out this rendition of Keane's "Somewhere Only We Know." Ah-maz-ing!
Check out their YouTube channel where they have a whole section devoted to wedding music. I'm sure you'll find songs you love! Which one is your favorite?