i think we cannot enjoy dolby digital or dts 5.1 in 360 unless we buy hdmi cable. ito lang kasi may digital sound and video (in one cable) to feed avr/htib (with hdmi-in). in this set up you need to connect your tv to the avr/htib (with hdmi-out).
ang 360 kasi, unlike dvd players, don't have readily available digital out (coax/optical) and video out terminals, therefore it needs accessory cable that separates optical/coax connection from video connection. but i don't think meron kasi walang ganitong accessory sa xbox official website. the accessory cables available are those with analog audio output and video output (component, s-video or vga).
with regards to resolution, i agree kaya mag out ng 1080p ang 360, i think regardless of its manufacturing date. and 1080p resolution is possible if yung disc source is 1080p (such as hd dvd, in this one naman you need to buy external hd dvd player for 360). hindi kasi naguupscale ang 360.