Agree completely with rmathis about 5.0 vs. 5.1 for this particular purpose. USB passthrough on 5.1 is currently broken, or extremely unreliable, and I can attest to that after testing 5.1 on one of my nodes. That, our of your specs you listed, is the only potential hardware/non-hardware issue I see.
My lab is pretty extensive (for a home lab), and I run three XBMC VMs plus two VMs that I use for desktops (my wife and son) and pass through a total of 5 video cards to these VMs as well as running HDMI and USB over CAT6 for all 5 VMs. My son's VM has a GTX560 passed through to it and plays COD Black Ops, and other demanding games w/o issue. The HDMI sound I haven't had any specific issues with other than driver related items.
