the speakers/headphones will undoubtedly make the largest difference. but beyond that, the interconnects wont make too much practical difference in themselves, whether optical/coaxial/hdmi/rca/etc. its more the quality of the audio source in this case, compressed/uncompressed/lossless/etc. you do have the issue of electrical noise from inside/outside your pc possibly interfering with analog interconnects (60hz ground loop hum for example), whichcase a digital interconnect would be the way to go given that.
theres also the issue of whether your receiver/amplifier has higher quality dacs than your sound card, or vice versa. if its an add on pci/pcie card chances are it 'might' have higher quality dacs than your amp, whichcase analog would be the way to go over digital as the sound card would be doing the processing. if the amp is better quality than the sound card, as is often the case with onboard sound, then digital is the way to go, as the amp would be processing instead.
the way to go if you really want to find out is to test out the sound card, then return it if youre unhappy with it, depending on the stores return policy.
hopefully that helps some.