Here's a quick reference guide I always give people...
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$30: (headphone): Asus Xonar DG
$50: (surround): Asus Xonar DS
$90: Asus Xonar D1(x)
$110: Auzen X-Fi Bravura (works especially well for headphone users)
$150: (surround): Auzen X-Meridian
$150: (headphone): Auzen X-Fi Forte
$190: (headphone): Xonar Essence ST(X) [Only if you don't plan on doing a surround setup]
$200(240): HT Omega Claro Halo (XT) [Halo if you use headphones, Halo XT if you want analogue surround in addition]
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I'd really recommend the X-Fi Bravura, like Davcon said. It has an integral Headphone amplifier and replaceable OPAMPS if you'd ever want to improve your sound quality for fairly cheap. It's really a well-rounded card in general that does everything fairly well.
If you can't stretch your budget that far, my second choice would be either the Asus Xonar DX or the Xonar D1 - the same card, only on different interfaces. You can decide which would suit you better. The only exception to this would be if you're a headphone user. If that's the case, then the Xonar DG would be a better choice as it also has an onboard headphone amp that you'll definitely want. Drivers for all the suggested cards are rock solid - please, don't buy a Creative Sound Blaster card... their drivers are absolutely awful and you'll be terribly disappointed when it only works half the time.