Just a quick Q, I'm currently fighting with my X-Fi (keeps not setting the correct master sample rate), and, well... given I only use Coaxial, so I even need to use the X-Fi? My motherboard has an onboard HD Audio chipset, wouldn't that give identical sound quality, assuming digital is digital and it supports KS or ASIO?

I would expect identical performance from the internal chip. Most soundcard quality variation is on the analog side (and the "weakness" is usually noise). The coax (S/PDIF?) connection is digital. It should be immune to noise, and there are no A/D or D/A converters required.

If you're searching for bit perfect playback, you should check whether or not your onboard chip resamples (chances are that it does). If you don't mind the resampling, go for it.