I pretty much based my Mozart HTPC purchases on the "failures" of my first custom-built HTPC (Gigabyte MB, AMD CPU).
You know how "picky" computers can be. I mean they seem to have minds of their own.
If I have sucessful confirmations of certain MBs + ATI Radeon 5xxx, I would stick to those.
I guess I could have picked another Gigabyte MB, but I did not want to take a chance. I don't think you would want to either, especially with new Radeon 56xx cards that are not even out yet!
You might check other forums that talk about the Radeon 5xxx cards and see what MBs people are using.
I could have picked another AMD CPU, but I wanted to try something different.
I guess when you have something "failed" the first time (Asus Xonar crashing on Gigabyte/AMD HTPC), you tend to shy away like 180 degrees.
Of course, that is just MY OWN personal experience.
You could try the Asus AMD + AMD Quad-Core CPU and tell us all of your great, literally COOLER experience (as oppose to my really, really literally HOTTER Intel Quad Core experience)!