PC Audio starting point

I can play iTunes with my PC using a modified Transit USB to S/PDIF converter with coax digital out. I drive a modified P-3A DAC. The quality is as follows:

.wav files @16/44.1 - as good as the best transport
iTunes - more dynamic than .wav, but a bit hollow on vocals
.wav files @24/96 - More dynamic than iTunes and more natural on vocals - leaves all transports in the dust.

What kind of PC do you have, a notebook or desktop/tower design? Also, can you describe the types of I/O options you have now with your PC (USB 1.1, USB 2.0, FireWire, etc.)? If you have a desktop/tower machine, do you have an available PCI expansion slot for accommodating a soundcard? There are many options, so any details you can provide would be great.

Many audio cards have toslink output. My motherboard has
toslink audio out and I'm using that into an outboard DAC.
I haven't had any troubles or any (noticeable) jitter.
Can't really recommend any particular audio card.
I also use an external USB DAC/ADC from Edirol
(www.edirol.com).
I'm using the entry level unit, but they make others
which may be better.
The sound is good, but not as good as using my
Musical Fidelity DAC and toslink.

Good luck
TJ