Hi Paul,
Yes you can fit the server with multiple cards and then stream to the different clients. Wrt your question on what to use as clients I would not go for too slow machines. If you run HDTV it will require a lot of CPU power to decode, and that is done on the clients. The alternative is to have a powerful gfx card and use HW acceleration to decide h264 streams (HDTV).
I use WinXP and my impression is that it works a little better than Vista atm. Perhaps it is just my impression, since I use XP myself so I can not give an absolute answer on that.
The central server does not have to be very powerful, at least not for TV, as the TV-stream will just be forwarded or recorded as-is and the power is needed on the client-side as I mentioned above.
/Peter |