A black suit works fine. As long as you're not in jeans and a blazer, you're fine. Most 'celebrity' men don't wear tuxes anymore, anyway. Just checkout the past decade at the Oscars or the Tonys (generally, the 2 'black tie' awards), and most men attending are not wearing a tux, so don't fret.
Due to the show being telecast 'live' they need everyone seated and ready for broadcast WAY before 8pm. 7pm gives them more than adequate time to get everyone seated and those late-comers celebs in their appropriate seats, etc. If they didn't say this "doors close at 7pm" -- you would see madness going on in the lobby, plus people scampering at 8:10pm rushing to get in along with people still walking around everywhere come 8pm when the show is going out 'live' around the globe. At 7pm, if you're not in...you ain't getting in. Period.
NOTE: I'm attending the show again this year and from past experience, under NO circumstance should you get there at 5pm (you'll be kicking yourself later as the evening wears you out). Getting there at 6:30-6:45pm is more than perfect. The less time you accumulate at Radio City Music Hall, the easier the night will feel. Trust me. It's going to be a LONG night. Eat BEFORE you get there because you're going to be STARVING mid-way during the evening. Again...trust me on this.
Have a GREAT time and rememeber -- please eat before the show. You have NO idea how hungry you're going to get. It's going a long night.