I am building a computer for my parents and they want to use it to watch movies. I have never hooked up an HDTV to a computer and am wondering if I can hook it up as though it was a second monitor and have it as a clone to their regular LCD. Also, by doing this will I need a beefy video card (like an NVIDIA 280 series)?
Some of the newer video cards have HDMI outputs, if you're planning on using the TV's speakers. Just go to Newegg.com and search for HDMI video card. For example, the ATI 4870 from MSI has an HDMI output as well as the usual RGB 15-pin D connector. I suggest you stick with a digital output to the TV rather than analog RGB, assuming your HDTV has both types. Blu-Ray has HDCP copy protection but that shouldn't be a problem, although you may have to set the TV as your primary display in Windows Vista or 7, instead of cloning the image as you mentioned.
You should set the TV input to dot-for-dot (no scaling) and may have to adjust the overscan too, unless there's a PC setting, so that the image exactly fills the screen.