If you think having more than one DIMM is going to have an effect on the sound, then surely a lower end card assembling the S/PDIF stream is going to have a massive impact.
Honestly, beyond the nice interface, I think this whole build is a waste. If you have a nice external DAC that can reclock the stream, it doesn't matter how much jitter enters before there.
I have a small project studio in my bedroom. I have an external Focusrite sound interface. I can't imagine anything sounding better than it. Or more precisely, I highly doubt I would ever be able to hear any improvement above it (heck, I could probably go down a couple levels and not hear any change, but this one had all the actually useful features I needed). This goes for any very well designed audio interface.
