I have been running Vista Media Center for a year now, and have just tried Windows 7 Ultimate with Media Center for kicks. There seems to be a few tricks to getting 7 to play nice, but all in all not bad.
I did a lot of research to finally build my htpc. Some of it was trial and error, pure and not so simple. I have tried MythTV, MythBuntu, Media Portal, GB PVR, BeyondTV, Sage, and others. I eventually tried Vista Media center and kept it. I put up some info on my website, along with links to where I found some good hardware suggestions, etc.
http://www.rumblegate.com ...
I found that you have to get a good balance between cpu and gpu. My vista box is an Athlon64 5200+, with an NVidia 8500GT passively cooled video card. The case is extremely important in keeping it cool, but it also has to be quiet. I chose an Antec case for that.
I am testing out an AMD 4850e cpu paired with a 780g motherboard, running Windows 7. So far, it does just fine. I do not have a bluray drive though, so I cannot comment on it. I have found OTA HD content to stress the system though, and a lower end low-power athlon would not cut it with the 780g. Maybe if it had the sideport memory, it would be better.
Anyway, here are my HTPC stats:
http://rumblegate.com/?page_id=9
Here are links to HTPC resources:
http://rumblegate.com/?page_id=36
When the wife gets into the HTPC, and stops using TIVO, you know you have a good system. It did take her a while to do that, but we are now selling it after being a TIVO customer for almost 5 years...