I recently upgraded my main Media Center to a Phenom II x4 940 from an AMD 5000+. Looks like AMD have finally got their act together on the Phenoms and this runs really nicely (5.9 in Vista) and cooler than the 5000 did.
I did have a couple of real gotchas though. When I first booted up I had an error message from Windows Media Player saying its network sharing service had errored. Then when I ran media center all was fine until I tried to play back recorded tv or view live tv when I just got a crash.
My immediate thought was a problem with the chip or the memory I had installed at the same time so I ran all the diagnostics and found no errors.
Suddenly it dawned DRM - a lot of DRM systems use part of the Chip ID to issue keys. With such a major change in chip this surely would have changed.
Doing some digging I found http://support.microsoft.com/default.aspx/kb/918218
Following the steps in there to reset the DRM store worked and I could play my recorded tv, watch live tv and no longer had a Windows Media Player issue. I did lose the drm license to one piece of content but that was not an issue.
Note if you use iTunes it is also going to think you are have a new PC so you will need to license your content for that PC
Next issue Cyberlink PowerDVD Ultra wouldn't play Blu-Ray disks. More to the point it would show the Menu but wouldn't play any selected content. I assume this is DRM again. The only way i could work round this one was to uninstall PowerDVD and reinstall - fortunately I still had my license key.
So a can of worms being eaten by a flock of pigeons but all now sorted and my PC is incredibly responsive so worth all the effort.