Where/How can I further improve PC audio quality?

I think Mesch has given you very good advice. In digital playback the clock is of paramount importance to reduce jitter. A computer is an electrically noisy environment and generally a bad one for a precision clock. A USB-to-S/PDIF converter with asynch USB (probably they all are) will have a precision clock housed in a more conducive environment and the asynch USB will have the computer clock slaved to that of the converter.

This will likely be the biggest bang for buck improvement. Going to higher resolution audio might be second. If you were to go with CD quality then the choice of playback software would be significant too, and the computer itself.

We are suggesting reclocking MP3's? to get to what? an MP4?

To me, at this time, my Spotify Premium subscription more than satisfies. I'm delighted with their solution to streaming audio and the simple to use GUI. The sound quality I have achieved so far is close to CD playback, however, I'm attemting to "tweak" out the best it can offer. The solution I'm going to try is a better card that has only digital outputs and claims to support asynch for improved clocking and a claimed low jitter output.

Thanks all again for your help!

The advice for a good USB converter is sound, however it requires that you jump through some hoops for the computer.

Rather than doing this, Just add a Synchro-Mesh reclocker to the digital feed from the PCI card and drive the DAC with the SM. Reducing jitter is what is needed and the SM will do this for you. It will even improve MP3. Relocating the clock that is used for master clock from the computer to a more controlled environment is the ticket. The SM does this for you. you can also upgrade the power supply to a Dynamo, which makes an even bigger difference. The SM can be used for reclocking your PCI card, a Sonos or even a CD transport. Reviews and customer feedbacks:

http://www.empiricalaudio.com/news-and-reviews/synchro-mesh

Steve N.
Empirical Audio

Are you looking to upgrade PC parts?

I upgraded from 4GB to 8GB, laptop to custom build desktop.

From dual core to 8 core CPU.

The difference was beyond belief! 1000% times better as playing HD Audio can eat at CPU usage etc.

Better PC parts better performance!