The only difference in those receivers is that they have the ability to decode the TrueHD, but your XA2 already does that and as you suggest passes via 5.1 analog.
As far as speakers... a 5.0 setup for $800 may not be much of an improvement over what you have now. I would probably start with the front LCR to start with so that you can upgrade better. Does HSU offer any towers and/or better center models? I've been looking for something for our great room and there seems to be just too many to choose from.
