if you want hd audio you'll need a hdmi output from the pc that can output 7.1 LPCM (nothing can from the pc atm) or 7.1 analogue outputs. since analogue is the only real way right now, you should be looking for the best quality analogue card you can find. the two cards at the top of the game are the x-fi relude and the xonar D2. the xonar d2 is cheaper and just edges out on the analogue output quality. the prelude is more expensive, is based on an x-fi and its drivers(both a pro and a con) and slightly inferior outputs.
if i were you id go for the cheaper card. well, i did anyway

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