Discussions

So I'm going the opera in a few weeks - I've never been before and I have no idea what to wear!  
 
I've heard that it's super formal (women in floor-length dresses, men in suits), but I've also heard that you can wear pretty much whatever you want (as long as it's nice) without feeling out of place.  
 
I'm 25, female - have a knee-length greyish dress (tight to the body), v-neck with short sleeves. Is this too informal?  
 
Has anyone been to the opera? What did you wear? What was everyone else wearing?  
 
Thanks for any advice!