What is appropriate attire for Broadway?

My girlfriend and I are going to NYC to see Billy Elliot at the end of December, and I was just wondering what would be appropriate to wear? Don't get me wrong, I've been to Broadway before, but all of the shows I've been to have been matinees. Also, this is the first time I've ever been to this venue (The Imperial Theatre). I don't know whether there is any sort of unspoken rule about upping the fancy for an evening show or about this particular venue. Can anyone help me out? Thanks.What is appropriate attire for Broadway?
What's the worst thing that can happen? You look less like a bum and she wears something sexy. Nearly all chicks enjoy dressing up. So why skimp?What is appropriate attire for Broadway?
Since it's not a matinee, wear long pants (it's winter too so it's going to be cold, we hit 19 last night on Long Island it should be somewhere in the 20s to upper 40s this time of year.) and a button down shirt. If you have a tie wear it, if you have a nice blazer instead wear it, but don't do the tie and blazer unless it's a suit. For your gal-a long dress with a nice sweater or shawl to keep warm in the theater and bring the heels in her bag so she can walk around the city without blisters (trust me I learned from experience). It's always better to be over dressed then under dressed.