ASUS Xonar Essence STX
On ebay, unused from a seller with 100% positive feedback. Bid is at $65 at the time of this posting, and it has a buy it now for $100. Great deal.
This is one of the best sound cards money can buy, one of few "audiophile meets gamer" sound cards. Great sound quality and it has a built-in headphone amplifier, as well as replacable opamps. The only downside is that the only virtual surround solution is Dolby Headphone (which is one of the worst), and it has no EAX support.