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Most Media Centre PCs have pretty good sound cards as standard. If you're only planning on running MP3 files the standard sound card should be more than good enough - for movies most MC PCs have a digital output to feed into a home cinema receiver/amp.
depending on your budget there are three soundcards i really recomend. i use a lynx l22 soundcard in my media pc but its not cheap. they sell new for around £600 but if your lucky you may find one on ebay a bit cheaper. i blew £2000 on my pc and use it as a music server which replaced my £1400 cd player. it sounds that good! no joke! your other answers are a auzentech x-merdian (£120) or an m-audio audiophile 192 (£100). also look at these pc cases (hifiatx.com) i use a hfx classic for my media pc and it completely silent. good luck with your media pc.