Modulate Hi-Def signal from HD-DVR or HD-DTV receiver
An old modulator (which is NTSC) will not be able to modulate a high-definition signal (which would need to be ATSC). The best you'd get would be a modulated SD signal.

Rather than a modulator, I recommend an HDMI splitter, and long HDMI cables. I bought mine from monoprice.com, I highly recommend their products.

My current configuration is a DISH receiver and other equipment in a closet, which then goes to this HDMI splitter, which splits to two TVs via two 40-foot HDMI cables.