Another option to mention is going optical to an external processor. This would alleviate some of the issues of the OS degrading sound quality. On optical out, the PC has no control over the volume, and bypasses the issues in the next paragraph. It is up to the processor (I'm talking something like the RF 3sixty, or pro-grade processor) to handle the volume then.
Something that not everyone knows is that in Windows, for digital music playback (i.e. MP3, WMA, FLAC, WAV, etc.) you should open up your volume control and set your WAV volume to 100%. Anything less and you are getting a bit-loss, because that is how the OS actually controls the WAV volume is by systematically removing bits from the sound stream! The master volume is handled a bit differently.
In theory, SPDIF to processor, even with an on-board optical out should be a VERY good option. You have no digital-to-analog-to-digital-to-analog conversions... You simply stay digital, until it leaves the processor at line level to go to your amps. Now, if you went with something like Zapco's DSP-6 or DSP amps, you could remain digital until the signal leaves the AMP!
BUT be prepared to drop some Benjamins for this option.
edit: nix the idea on the 3sixty... Doesn't appear to have digital-in.