well one thing that may be in our favor is that myth is being ported to windows.. if that hd capture card only has a windows driver, myth will probably have a windows backend available in the next year anyway. The development of the frontend is going along pretty briskly right now while I've been keeping track of it
Actually, that seems to be against us, if we want a Linux backend and not a Windows one. In general a Windows MythTV seems like competition for developer time to Linux development for it, so repurposeable development like Linux drivers for video cards will sometimes lose to Windows-only development. It's possible that the Windows version will attract Windows developers to MythTV and draw some of their larger community to the Linux version, "biplatform MythTV as OS crossroads", but since Windows developers are both on average less skilled, and overall much more skilled in only "Windows" and not as much in general "development", I think it's a net loss to Linux.
I think that analysis is especially true for HW vendors like Happauge.