I have hooked up my Panasonic Viera 720p/1080i HDTV to the following cards:
Nvidia 7600GT - via. VGA + Component. DVI (7600GT) to HDMI (TV) did not work, IMO an HDCP handshaking issue.
Nvidia 8800GT - via. VGA, Component and DVI to HDMI. All worked fine.
ATI HD4870 - via. DVI to HDMI. Works fine.
If its for movies only, find a nice card with HDMI or DVI and some hardware acceleration for HD content and you'll be golden.
For gaming, any card from the 8000 series from Nvidia and the ATI equivilent onwards should be fine as far as HDTV's go. You may wish to consult as to performance, heat and noise considering your PC's operation.