Given that the MB has been available for all of one day now, you'd kind of be blazing a trail on this one.
1. If you want to go Vista, premium is probably fine.
2. Vista 64 is probably the least supported OS for HTPCs in terms of helper applications. The chipset shares the main memory for graphics, so 4G, if you are using Vista, is probably a good idea.
3. The GPU on the 780 will hand BluRay. That CPU could probably do it in S/W.
4. Do you mean 120Hz refresh rate? I don't think the TV will handle that.
5. Improvement would be for games only.