DAC vs CDP DAC vs Processor DACs?

I know the source plays a huge role in how the (Perhaps a parasound D3) would have over say a Halo C1/C2 processor for 2Ch audio? How about external DAC's?

I know even low end receivers these days are coming with 192 kHz/24-Bit DAC's, my current 5760 by yamaha has them. What makes a higher end DAC that much better? I assume all of them interpolate the digital signal and manipulate the analog output according to the interpolated data in a similar fashion.
I don't know for certain how stand alone external dacs achieve a better sound than internal ones, but my external 192/24 dac is definitely superior to my players internal 192/24 dac. I think part of it is the power supply and the isolation from other digital / rf interference. And of course quality of components.
Do you know if the external DAC's will allow digital passthrough for sources that can do more then 2ch? How many of then can decode DTS/Dolby Digtal etc?