I don't think there's any HDMI cards out there. They just output in DVI, which is the same thing. You could just get a DVI to HDMI converter, however.
Really, if you're looking for a fantastic HTPC video card, take a look at the ATI 2400/2600 series. They feature HDCP (which the Nvidia 8500/8600 doesn't), H264 hardware decoding, and VC-1 decoding.
ATI will be coming out with the cards sometime in AGP but for now they're just PCI-e
Too bad nvidia has dropped support for AGP already.
Of course, if you don't mind not having HDCP support then the 8500/8600 series looks good as well.
Read the review here
and decide for your self.
Yes, HDMI isn't totally necessary
On a final note, if your PC can handle the videos on its own without hardware acceleration, then just picking up any old card out of the last generation will do you good. A 7600 sounds good, a 7900GS sounds good as well. However, I'm no expert in these things, but I am capable of playing 720p content on my P4 2.533ghz with a 6600GT in it.