I have a PC down in my office running Vista and an Xbox 360 extender in the media room. What I don't have is HD of any kind on that setup, so I can't answer your question directly. There's a lot of complaining that the extenders don't support some of the more popular codecs such as DivX, XviD, etc. For HD recording, I'm guessing good codecs such as those are a must, lest your TV shows take up 50Gb each.
There's quite a bit of information about HD on Vista Media Center and an equal amount of complaining. In all the material I've read, I've not read about someone using an extender to watch recorded HDTV, but that doesn't mean it can't be done. Some people are having good luck with a product called HDHomerun (I think) and Media Center, but again, no mention of extenders. I was going to try it out but, lucky me, my cable company took all of their unencrypted HD channels off the air this past weekend. It's OTA for me or $$$ and another box in my AV Rack.
I'll be interested in seeing what other people say. I am virtually in the same situation as you at this point.
