The ATI card that does HD is the ATI HDTV Wonder, and while the card is capable of doing SD & HD, I'm pretty sure I read somewhere MCE will only allow it to install as one or the other.

So, if that's correct (I do not run the HDTV Wonder as I have no HD source near my PC) and you want both, you really need a TV Wonder Elite plus an HDTV Wonder. The bonus is you'll be able to record one HD and one SD show at the same time. I hear they can be a bit of fun to install under MCE but it does work.