the xonar STX may be overkill, its fantastic though. If you want a better sound than the motherboard with those headphones, you don't need to get crazy. My personal choice for those headphones, whether for music or gaming, would be one of these two:
Well, at 55 Ohms impedance, you might see more benifit from a soundcard with a built in amp, so the ASUS Xonar DG might be the way for you to go...DG, DS, or DX are probably your three best bets in any case.