I just bought an Audigy 4 (non pro) sound card, and I'm quite happy with the sound quality. It has a S/N ratio of 106dB, and in stereo mode it uses a 24bit/192kHz DAC. I'm not sure how it compares to the Yamaha receiver in sound quality.
Another thing to consider... If you use the DAC on the Yamaha, you have already made your choice of which amplifier you intend to use - the Yamaha. If you get a quality sound card, you can get another higher quality amp, and therefore higher quality sound, which is I assume your ultimate goal....