the 777 is a good-sounding player (I used to have one) - you probably would need to buy a kilobuck DAC (plus a good interconect...) to hear a significant difference. if you can't audition first, I'd say don't do it.
as far as spending $650 to improve your front end, you might want to spend it on good interconnects for your 777 if you don't already have nice ones, or a power conditioner - I think that power conditioning has the most effect on digital.
as far as spending $650 to improve your front end, you might want to spend it on good interconnects for your 777 if you don't already have nice ones, or a power conditioner - I think that power conditioning has the most effect on digital.