If keeping the HTPC fan noise quiet is important to you, don't put the GTX580 in there.
There are loads of quieter, cooler-running options but for single-screen 1080p gaming you'll probably find something like the vapor-x 6850's more than adequate.
I'm yet to upgrade my GTX460 on the HTPC because it's giving me fluid gameplay at some fairly high detail levels, but even that old card passes audio over HDMI.
As for issues with HDMI audio, I've not encountered them, but it might be partly because my TV and surround sound system use HDMI-CEC passthrough and are working well together, so the 5.1 audio device is detected by windows.
Every other HDMI audio solution I've encoutered just works, but I've never gone as far as checking surround sound capabilities, since it has almost always been TV's rubbish little stereo speakers.
I hear rig lists are all the rage, and I <3 the rage! Workstation = Black tower thing; HTPC = Shhhh!; Laptop - AMAZING FOLDING PC!