If noisy PSUs were causing actual software processing errors, nothing in PCs would work (not the OS, not audio, nothing). The more usual concern is that electrical noise from the PC can sometimes affect a poorly insulated DAC when the two share an electrical connection.
But with an optical connection and a quality DAC, I don't think it's possible that a noisy PSU could make any meaningful difference. An optical digital connection does not electrically link the PC to the DAC, so there can't be direct electrical hum or interference from a noisy PSU.
A hostile electrical environment in the source PC can (at least theoretically) create timing errors (called jitter), but there are two things to consider:
a) There doesn't seem to be much hard evidence (to include measurements) that noisy PSUs inside modern PCs actually cause meaningful (meaning audible) increases in jitter (over the baseline jitter that would be caused by operating a PC with any PSU). There are papers documenting the issue generally and a few blog/forum posts by DAC designers talking in very abstract terms, but not much by way of measurement on modern equipment (the only measurements on this specific issue that I've seen were in some AES papers from the 90's). To be clear, jitter is real and measurable, and can be audible, but it's not necessarily easy to hear, and it's not clear that noisy PSUs in modern equipment affect digital outputs (like TOSLINK) in any meaningful way, or that a linear supply would be any better. There may well be a measurable effect, but it won't likely be an audible one. But none of that matters very much because:
b) Decent modern DACs are highly resistant to jitter, so if it concerns you, just make sure to find a DAC that has solid (measured) jitter rejection performance. A DAC with 110dB of jitter rejection and an optical connection simply doesn't care what your PC is doing in any meaningful way.
Here's a good article on the issue, with measurements of a few differently priced DACs:
http://nwavguy.blogspot.com/2011/02/jitter-does-it-matter.html.