I heartily recommend the M-Audio Revolution for great 2-channel sound (haven't tested the full 7.1), it beats any Creative product hands down in sound quality. It will even drive cans like 600's reasonably well right out of the output!
Source defenitely matters. As you get better amps and more revealing headphones, they will only amplify the defficiencies in your source. They might also reveal the defficiencies in the media you choose to play as well. Garbage in, Garbage out.
They have a card by Sek'd (German) called the Prodif Plus that sounds pretty good. It is more of a pro/studio use card. It has onboard 20bit ad/da converters and comes with a nice .wav player demoware software program. Their software does *way* better than Windoze Media Player or cheaper stuff like Winamp at decoding stuff in .wav format.