Yeah, for some reason some of the high end Nvidia cards don't bitstream HD audio over HDMI. The 570 and the 580 can't do it. Most/All of the current lower end Nvidia cards can bitstream. You'll have to check the specific video chipset to see if it bitstreams HD audio.
All of the current AMD cards bitstream over HDMI though.
So amd cards then... i was thinking between the HD6670 or the HD6570... would it matter if its GDDR3 or a GDDR5 is better? Also, i' ll probably do little gaming on the htpc (League of Legends)