The object of booting the system with only the HDTV connected was to get a valid display. From there, check which resolutions work and which don't. Each time you change the display resolution, Catalyst Control Center generates a 15 second timer which allows you to either keep or disregard the new resolution setting. If no action is taken (as would be the case of an Invalid Format where you cannot select to keep or disregard), your system will automatically revert back to the valid settings.
Using this process, find a resolution that works on your HDTV and matches or comes close to your monitor's recommended resolution. Once your system is stable connected to you HTPC, shut it down and reconnect the monitor. You system should boot to the HDTV again. With the second monitor attached, you should be able to select either Cloned or Extended modes to activate the monitor as a secondary device.
Once both displays are active, then you can change the monitor to the primary display.