In this situation I personally would get an amp with several HDMI inputs and one out. Then you use HDMI to connect all devices (HTPC, Xbox, and TV receiver) directly to the amp. I would also run an optical to the amp from the TV so when the TV's tuner is used it also benefits from the better sound.
As an example something like
Sony-strdh500
As I understand it the problem with using a home cinema is not just that the sound quality wont be up to the same quality, but some TV sets downmix a 5.1 signal they get in, to a 2.1 they output through their optical, rather than using the superior audio processing of the HTK to downmix.