Microsoft will never call the next version Xbox 8. Not only would it be completely wrong sequentially, however also bear in mind that the next Xbox probably will stick around for much longer than what the current Windows 8 and WP8 will. The Xbox 360 is with us since before Vista times remember....
Not so sure about the Infinity branding either, as quite a few companies are associated with that term.
I take anything but 720 however, would be ludicrous!
Personally, I think Xbox Infinity in feasible given the logo looks like an 8 on it's side, and the new Xbox is rumoured to run a modified version of Windows 8 under the hood.