Ok, you have a couple of options. I will just mention a few when connecting your PC/Mac to your G68.
Since you mention soundquality over and over then I will not mention the "ghetto" alternatives. ;-)
* Lynx AES16e PCIe card - Will output PCM to your G68 via SPDIF, VERY clean signal, low jitter! Troublefree!
* Ghetto alternative: ATI 4850/4870 HDMI GFX and soundcard. As ghetto as it gets with driverupdates and such, but I know you can get atleast 2ch 24/96KHz PCM out of it and it will go straight into your HD621. :]
Ok I did mention the "alternative" which I will probably try it's next incarnation in 6 months.
