Assuming the TV has a HDMI adaptor you can use a DVI-HDMI cable or a DVI-DVI cable and an adaptor - the connectors are electrically compatible.
If the Video card does not support HDCP, it will still be able to use the TV as a monitor. Games and DVDs will play in full quality. If you add a Blu-Ray or HD-DVD drive in the future however, these will be "downsampled" when played without HDCP.
You could then use the TV as a monitor. I'm not quite sure what you mean by "Could use HDTV", you are using your HDTV no matter what the source... and connecting a PC to your HDTV isnt going to make your TV programs Hi def... :s