A few comments:
Re: Video Display
I second NorwegianDawg's comments on the TV issue. The only reason to go with the Plasma is if you can't use a projector and you want to wall mount the display. If it's possible to put the TV on a stand, the DLP option would be the way I'd go (IMO). It might not have the sex appeal of a plasma, but it'll cost less, last longer and give you a better picture. All of this is moot if wall mounting is what you really need/want though.
Also, I wasn't clear if there was something you were still confused about regarding the HDTV stuff.
Re: Receiver
That Marantz is a great piece of equipment. No worries there. The fact that it may or may not have HDMI switching capabability can be problematic, but there are always solutions. I have an older Denon unit -- no switching capability. Fortunately, my TV has 5 inputs to accomodate my various video sources; however, if/when I upgrade to the new DVD format, I'll have to get some other video switcher. Maybe something like
this would work well.
Re: Pipe
I think the idea about running a pipe for future/potential cables is an EXCELLENT idea. In fact, I'd be doing that for all of the in-wall wiring you're going to be doing, just in case you have to later switch out the wires for something else anyway.
Re: Accoustic Treatments
IMO -- very important to improve the imaging/sound/etc of your system. And since you're in the building mode, they're really easy to make, and don't cost a ton of $$. Here are a couple of sites that show you how to build them.
DIY Panels
DIY Panels Also
This second link I think is more relevant -- newer information and it's more specific about the fiberglass to be used -- Owens Corning 703 is the standard, but there are many other manufacturers with the same specs. The trick always seems to be able to find the fiberglass -- it's not something you can generally just pick up at Home Depot.
Also, be warned, too much treatment will kill a room just as bad, or actually worse, than too little. You only need maybe 25% coverage. The spots I'd definitely hit would be the first reflections on the side walls and the first reflections off of the ceiling. If you're willing and able, I can expand on this.
Re: Room
That room is REALLY making me jealous.
Let me know if anything above wasn't clear and I'll do my best to clarify.
JCD