I agree that we should stick with what works for us.
I think it's largely dependent on the configuration. I had an Audigy 4 Pro in the past that was installed in a noisy PC (SLI + SCSI RAID setup with a Pentium D pushed to the limits in clocking and signalling levels). The noise floor rose considerably under load, as the PC ramped up clocks and voltages. I've tried three high grade power supplies on this PC, to no avail. Stock clocks and voltages helped, but there was still that audible noise floor. The Yamaha SW-1000XG seemed to have better noise rejection despite the lower specs (perhaps they had different design tolerances... plus the fact that the specs are probably written for just the board itself, without factoring noise from adjacent components/devices). Too bad the SW-1000XG wasn't able to handle 24-bit audio, and didn't accelerate consumer-level applications.
When I transferred the same sound card (Audigy 4 Pro) to an undervolted i7, it was very quiet (though I still wouldn't call its output high quality). I think the ASUS Xonar's shielding is a step towards the right direction, but it probably won't be as good as a decent outboard solution. I have an X-Fi Titanium and an EMU at the moment, but I just use their digital outputs to feed outboard ASRCs and DACs.
I don't see myself going back to analogue outputs from a PC, unless they build something spectacular (or if I really need to downscale).