If SACD performance is a high priority, I don't think I'd sell the 777 (ES ?) until I've heard what the OPPO will do with SACDs. I have tremendous regard for OPPO, but matching the 777 is a pretty tall order. I agree with Gonk on the 555, the OPPO should give you better performance.
There are solutions for sharing the 7.1 inputs between devices thougt they are either pricey (VECTOR) or in-elagant( 3 Radio Shack AV switchers , my gastly solution ). If a 2-channel set-up is anywhere in your future, the 777 would be great for that , It's hard to beat for 2 channel SACDs.