If you're using a digital output there should be absolutely no difference between a £10,000 cd player and an onboard sound card. Since more and more amps are coming with digital inputs, I can see little reason for an expensive sound card - except of course for gaming, and their various fancy film/surround/blahblah features.
It's all about the speakers and the room - you can fiddle about with the little, cheap-to-experiment-with things, but ultimately you'll spend the time trying to convincing yourself you're hearing improvements.