Well, for my part I can only say that when I plug headphones into my laptop or the desktop machine here at work, I hear pops, growls clicks and buzzes - probably from the drive and other electronics. Those same noises can be heard from my computer speakers when I wind the volume up a bit...
The M-Audio Audiophile 2496 would be a concrete suggestion for a high quality audio card with SPDIF. Will set you back about $200 I think. http://www.m-audio.com
However, you might want to look at cards with optical outs too since optical cables conduct no electricity. Isolating your sound...
I would disagree with Vince on this. An SPDIF (or optical) digital connection will keep the noise of the computer out of the receiver.
When I hook my laptop to speakers using the normal stereo output, I get pops, clicks and whines out of the speakers as well from the electronic noise inside...