the hd598 are fairly easy to drive and most onboard can drive them. they do gain a bit more low end when paired with a more powerful amplifier but thats about it.
i would try your onboard first. if the sound quality does not appeal to you or you hear distortion (buzzing, crackling, distorted sound, other noise) or it does not have the features you want (such as virtual surround) then i'd think about upgrading.
if you wanted to go with a soundcard i'd suggest either the xonar dg or xonar dx which are reasonably priced products. the xonar dg has a more powerful amplifier built in but the xonar dx is a higher end card. both have virtual surround support (dolby headphone). if you eventually planned on getting a dedicated headphone...