[rlj5242]

GrayWolf, will the multiswitch you linked to also work with HDDTV if I decide to get it next year?

No.

What about the Eagle Aspen 5x8 Multiswitch with Power Supply (S-4180-GX+) instead of the 3x8?

Yes. It will work with your current dish or the PhaseIII multi-sat dish.

Also Can I install the Multiswitch outside behind the dish?

Not recommended. Put it in a place that is at least out of the direct weather. In the attic is recommended.

But correct me if I am wrong; A diplexor would alow me to run just one cable to each room and then split it again to go to each tuner on the receiver???

You are wrong. Diplexers are used to combine satellite and antenna frequncies on the same cable. To do what you are describing would take stackers and de-stackers. They are expensive. Do a search because they have been discussed a lot.

-Robert