I have bought Asus M3N78-EM motherboard for my new HTPC. I'm currently testing it with KnoppMyth R5.5 and installation with KnoppMyth R5.5 worked flawlessly.
To enable optical sound you only need to edit
etc/modprobe.d/soundfile, it needs to have this line:
options snd-hda-intel index=0 model=6stack-digout
sound chip on this motherboard is MCP78S as seen via lspci -v command:
00:07.0 Audio device: nVidia Corporation MCP78S [GeForce 8200] High Definition Audio (rev a1)
Subsystem: ASUSTeK Computer Inc. Unknown device 82fe
Flags: bus master, 66MHz, fast devsel, latency 0, IRQ 21
Memory at fce78000 (32-bit, non-prefetchable) [size=16K]
Capabilities:  Power Management version 2
Capabilities:  Message Signalled Interrupts: Mask+ 64bit+ Queue=0/0 Enable-
Capabilities: [6c] HyperTransport: MSI Mapping
Enjoy your new digital sound.