I'm going with 4,5,1,2,3,6...I saw that suggested on anther forum and like the idea of ending with the true wrap up of the Saga, but maintaining the surprises from Empire.
As for audio and picture quality, I've watched 4,5,1 and I have to agree with ref head. The picture on ANH was miles better that I've seen it before, but overall just average picture and sound. Empire looked amazing and the sound was good too. Ep. 1 was great. The sound during Duel of the Fates was amazing. I can't wait to get to the ROTS, I've heard and read nothing but good things.
Definately worth the $80 to have these on blu ray, even with the changes. If they only had an option that would scrub Jar Jar from Ep. 1....