Hmmm... might want to look into getting a headamp, or even a USB DAC/amp combo (usually pretty pricey though for a good one). There are many factors that would compromise the sound quality of a dedicated add-in card (EMI from GPU fan, cheaper/insufficient/smaller components in the [amp's] circuitry, stuff like that). Also the amps built into most soundcards generally will not match the quality and offerings of a decent standalone headamp (for example, it might not actually be 'strong' enough to drive a demanding pair of headphones, might not be as transparent, etc etc). Of course, if you just get a dedicated headamp, you would still be using the faulty amp on the soundcard itself (unless you have a true line-out of some sorts); essentially double-amping the signal - something that is generally considered undesirable (dirty sound in, dirty sound out). This is where my USB DAC/amp suggestion stems from. At the same time, if you get a USB DAC/amp, you would lose hardware acceleration for games, and would probably see an (insignificant) increase in CPU load. On the plus side, you would be able to use it on your desktop, laptop, and possible even any portable source you might have. If you really are looking into getting a new soundcard, you would want to look for something that has a fairly decent DAC along with decent op-amps and stuff - essentially something with a respectable SQ. It's really not that bad an option for you since IIRC, HD595's really aren't that hard to drive (so long as the amp is clean, it should be sufficient). I know I sound very vague and seem to be generalizing stuff or even contradicting myself. Truth is, there is a plethora of choices available to you, and you will have to do a bit of research to figure out what suits your needs the best.
This or
this might be a good place to start... oh and sorry about your wallet
