Any HDTV 720p signals should be converted by your STB to 1080i for your set. As far as HTPC is concerned, you should look in the Home Theater Computers forum. There are many "fixes" for getting your PC's video card to match the capabilities of your set. Often, you setup your PC to output 1080i or 540p (same scan rate as 1080i). Simply giving any HDTV a standard PC resolution like 640x480 or 800x600 can cause problems, even if they do support 720p. These are not typically like PC monitors which are able to sync to a wide variety of resolutions/refresh rates, they only have a few supported configurations and it is important that you make your PC bend to the will of your TV and not the other way around as improper signals can damage the TV.
Of course, if you get a digital projector, you avoid much of that because most of them (maybe all) will support common PC resolutions.
jake