I always thought that the quality of analog output rested mostly with the DAC chips on the sound card, were as those mods seem mostly to center around conditioning the power delivery.
Better DAC, better sound. That's about it.
Personally, for the best quality, I just did SPDIF-OUT for a full digital path from my audio file being decoded into my Sony "home theater" stereo receiver then hooked my headphones up to that. I don't need a fancy card for that... Intel-HDA works as well as anything since it does zero processing to the sound at that point.
In fact that is quite a bit better then my old Audigy. As part of the hardware mixing features of the sound card the Audigy remixed the audio stream to it's prefered format no matter what you did.. unless it was Ac3 passthough or whatever.
Well the electrical magnetic radiation is trapped in your computer case by fact that it's a metal box. Every little circuit and clock and proccessor in your computer is just screaming with radio waves. And then there is a crapload of fast signaling going around on the cables inside your case for the harddrive connections and whatnot. (of course the radiation going into the case and into the sound card is going to leak out the coax for your audio output!)As for RF energy, the various clocks on a sound card operate at frequencies up to the MHz range. Without proper filtering, these clocks (and their harmonics and subharmonics) can leak through the DACs into the output signal, and then be emitted into the air by your cables (which act as transmitting antennas). You can check for unwanted frequencies on the wire using a high bandwidth spectrum analyzer. You can also go to the expense of building a shielded room (my previous employer had one), complete with RF test equipment, if you're feeling a bit crazy.
So the radiation is going to have the biggest effect actual analog circuits of your sound card then on the wiring on the outside of the case. So any sort of analog circuit inside the computer is automatically at a huge disadvantage and is subject to all sorts of problems.
You could probably make a metal case around your sound card, but it's going to be difficult to make sure it's properly grounded as much as possible so that it doesn't act like a great big antenna. (copper tape works wonders)
This is the main reason why, for best sound quality, I prefer to run a digital output to a external receiver, which then does the digital to analog transformation. But I still wouldn't mind a very very nice audio card.
Probably the best sound for a mobile system is probably a high quality USB sound card. Sure you have high latency, but that is not important for playback. I don't have one to recommend, right now... most usb audio is cruddy, but there are some nice ones out there.