I think the 5xxx series are actually a better fit for mythtv. Newer cards are only faster for 3D really, (which doesn't really help mythtv). Hardware mpeg 2 decoding for nvidia cards is done with XvMC (like purevideo), and it works well on any card 4xxx series or up. Actually, the 6xxx series apparently doesn't support hardware video overlay as well as older cards (http://www.linuxis.us/linux/media/ho...tpc/video.html
) so why waste more money on a card that's not as well supported?
Personally I use the 5200: you can get a quiet one with a heatsink instead of a fan and with DVI, S-video and VGA outputs, pci or agp. It's quite capable of 1080i and 720p (although I use it over VGA to a component adapter, never tried my DVI).
A few words of caution though: XvMC can be a pain with mythtv although it looks like its being improved. It can be simpler to just get a good fast 3 gHz processor to decode the mpegs and not push it onto the graphics card. Also, I've found it easier to send the tv a 720p signal (even though it's supposed to be a native 1080i set) because there's no trouble over interlacing and flickery menus, not to mention less of a strain on my hardware (but I don't have any hdtv programs yet, just analog tv and dvds upsampled to hd resolutions). If you are trying 1080i, make sure to use the 1.0-8xxx series drivers because older ones had a bug that resulted in a really messed up "split screen" effect. Hope some of this helps