I have SageTV running in my house. I have 1 HD200 extender and 1 HD300 extender. The HD300 is pretty nice because you can mount it behind a flat screen TV and use the remote IR receiver.
Some comments about SageTV (as a user).
1. The interface isn't as polished and pretty as WMC but it is getting better.
2. Sage to many people still has the feel that it is for enthusiasts (people who like to constantly be messing with their systems).
3. The Sage extenders (HD200 and HD300) work very well.
4. SageTV is not free. Many people say WMC is free since it is in Windows7.
5. I ran the previous version of SageTV as an add-in on my HP EX485 MSS. This was a little too much for the server in addition to it's other tasks. Now I have SageTV running as a service on my core i3 desktop computer. I don't even notice it is there.
If you have some specific questions about SageTV I would be glad to try and answer them. There are users in the UK using SageTV.
Yes, a lot of people were hoping for WHS/MC integration. But I assume you don't need to move off v1 right now, so why not pick up a Revo or something similar for use as an HTPC? Small, quiet, able. I'm going to suggest that you may not want a server in the living room, with all those spinning drives. As to 24/7, you could always use Lights Outs add-in to manage the on/off times.
trixbox? That's a VoIP phone system, right? Tell me why you're using that in your home? I'm curious.