Best option at that price is the ASUS Xonar
Essence ST Ds, although the Xonar D1/DX and HT Omega Striker can be had for just a bit more.
OP, give your onboard codec a try first.
Many people consider them 'good enough' for every day ussage.
If you decide the sound is to muddy for your taste, I would recommend looking into an Asus Xonar D1
(PCI) or Xonar DX
Both have top notch sound quality and very nice feature packages.