My opinion...digital is very robust and the quality of the interconnect doesn't really matter. Early optical cables were spotty in quality and there were biases against them. If your mother board's got a digital output, I'd use it. A good sound card is mostly for converting to analog or for giving more output options. If you have a receiver or other device that will accept the digital output I see no reason for a soundcard.

SPDIF is a standard RCA style output right? I use a cheap old generic RCA interconnect that came with some component I bought for my digital connections on my home system. It's perfect. Others will tell you they hear differences between opitical cables, RCA vs optical etc. I'll go on record as stating its their imagination they hear, or something else in their system settings. Unless you run your RCA interconnect through a microwave oven and over some magnets it well perform fine, no need to spend more money on something fancy.