the av-710 is unamped, it isn't made for headphones so unless you have an external amp, there is no comparison
the x-fi platinum has a mediocre amp same as the audigy2 cards, the x-fi elite pro would be much better
having an external amp would make your HD650 sound much better with any card
in regards to sound quality, i own the av-710 and have sampled the x-fi extreme music, they are both very good in the right settings (hi sample rate for av-710, audio creation mode with crystallizer off on the x-fi), i tested them on different speakers and can't say which is better, they are both very clear sounding and have good balance, you can't go wrong with either, if you're a gamer go with the x-fi extreme music, if you just want good sound quality the av-710 is great
i agree with 003, the revolution 5.1 would probably be the best choice for headphones without an external amp