I havent bought a sound card in a loooong time - but I know sound blaster has always made the best cards. I think its usually the choice for HTPCs... maybe check out the Z? (the Zx is a step up.. and the ZxR is the top of the line)
I would say the sound card is the least of your worries. A good Amplifier and solid speakers/headphones are the main starting point.
you could have an astro a50 headset... but if you are using the onboard audio, there is no point.
The starting point is the sound card.. since thats the source.
thats like getting super speakers in your car, but still using the audiovox radio