If coonan is right, that 750 fps and up is a firearm, with stiff penalties for urban use, then a .22 cal air rifle @ 750 is going to be superior to a .177 cal air rifle - more energy. If you can afford it, I'd recommend the Air Force Talon "SS" - the SS is the suppressed model - very quiet. I know you can get it in .177, and I think you can change out the barrel with a special order, to .22.
http://www.airforceairguns.com/
I have an RWS model 34, and it's loud, and it would violate the 750 fps rule you apparently have.