Welcome. I'm new to this too, and can say there's some great help to be found here.
For a few specific things, for sound isolation I also read about "Green Glue" (http://www.greengluecompany.com/) and they sounded quite interesting, and a few people on avforums seemed to have good experiences with it. Might be another way.
For the HDMI cable, the way I did it was to run a conduit to where the projector was so that I could change out the cable if necessary... The cheap Monoprice 35 foot cable seemed to pass at 1080p (http://gizmodo.com/gadgets/hdmi-cable-battlemodo?view=full) but it's one of those things where you might just have to test it with your own equipment. And then if the cheap one doesn't work then a more expensive Blue Jeans or something cable could be put in after the fact.
My small input, and post pics when you can!
