I picked up a cheap Xonar DG to replace my GA-Z77X-D3H onboard. The sound in games and music was improved significatly and justified the small cost, but it may have been more down to the headphone-amp that gave some extra juice to my high-impedance headphones.
If nothing else, the Xonar Audio Center has a few handy controls to play with so get the kinda of sound you are after.
If you go ahead with your purchase, I would be interested to here about your experiences with it.