I would also add that in the lower stratosphere of pricing, that a competent USB DAC will beat the SPDIF out of a flaky motherboard SPDIF, especially if you cannot set the bit rate of the motherboard device to 16/44.1. Many only output 16/48, which is DVD speed, having to upsample your file to 48, which will not make it sound better.
Mac SPDIF (toslink) outputs are better than the PC based motherboard SPDIF I have used in the past, but a clean USB is still better. I use the CEntrance DACPort with my Mac laptop, and it sounds better than the SPDIF to a similarly priced DAC. Huge difference? - no, but with good headphones I can tell a difference, and I am not that good.
Now there are competent audio cards with improved SPDIF, in fact there are several reviewed in The Absolute Sound magazine this month, some inexpensive, some closer to $500. They gave a nod to a card under $200 for the best SPDIF. Some of the difficulty with SPDIF at all from a PC is the driver for the specific device. Some are easy to set, some are not. Some are buggy and lock up a lot, some don't. Generalization is tough here, and some are system dependent.
I used to build my own PCs before I went Mac. Some of the drivers will drive you batty.
Now the USB-SPDIF converters are really smooth now a days to work with a DAC with only SPDIF, I use a Halide Bridge to a PSAudio Digital Link III, the USB on that DAC is very outdated. This converter easily beat the USB in the DAC, and Toslink from my iMac, easy difference to spot with a nice system.
