While there is no actual limit on HDMI, the theoretical limit is around the 45-50' range; quality cables can compensate, but don't ever try to join two cables - you must use one continuous cable.
What Pete said about Coax - I did 3 runs to each TV in the last house; although the advent of DTV's SWM has made my life SO much easier; even still I'd want 2 so you can run cameras or other sources using a modulator.
And I don't run 2 runs of Cat6 for just computer locations - I run them to every location. In each room, I do it to two opposing walls as well. For instance, most bedrooms have a bed & nightstand on one wall, and a TV on the opposite wall. I put a couple RG6's and a couple Cat5's to the TV wall, and two more Cat5's on the "Bed" wall for a nightstand internet clock radio or phone (VOIP or analog), or whatever.
AND any place you might want a security system keypad (each entry to the house and in the master bedroom) I install a Cat5 and an 18/4 - that'll support a speaker (great with an M1) or even other alarm panels entirely. It's good to have both.
It's doable - My brother-in-law and I did a 2500sq ft' house - a couple miles of cable - in a few nights after work. Granted, near the end, he slipped and crashed through my ceiling and vowed to never enter another attic again for the rest of his life, leaving me to repair the ceiling, finish the wiring, and reinstall the can light he took with him... but that's a different story

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And conduit is king - any place you can, run it! And don't run your wires through it now - run them along side leaving the conduit empty for future use.