Al,
I do use the Tascam US-144 as a USB to SPDIF converter. It was works well in my setup with great sound. As far as I know there is no d/a or a/d conversion with this unit if you are sending out via SPDIF. It does include analog out but I wasn't interested in using it as a DAC.
I was considering the offerings from Empirical Audio & Bel Canto but I decided to wait for a 24b/192khz model before I drop $500+ on a USB converter. So In the meantime I am more than happy with the US-144 considering its bargain basement price.
Interlochen,
My PC Specs are...
Intel E6300 CPU (considering a 35w Celeron 430)
XFX Nforce 630i Mobo (not the best board for audio, may replace)
1TB Western Digital Caviar Green HDD
AZiO ENC311SU31 HDD Enclosure
G.SKill 1GB DDR2 PC6400 (underclocked to 600mhz at CAS 3 Latency)
Corsair HX850 PSU (excellent low ripple & dc ouput)
Thermalright AXP-140 Heatsink
Omaura TF8 Aluminum Case (Company may have gone bye bye)
You should also check out the cics guide "The art of building computer transports" which you can find at
http://imageevent.com/cics/v03theartofbuildingcomputertrnsp I didn't follow the guide step by step but some of the recommendations helped me with my system.