The main benefit of the GSX is the surround reproduction - the SNR and harmonic distortion aren't really that amazing - fairly much inline with motherboard audio just potentially a bit more isolated from electrical noise generated within the PC.
The GSX IIRC you have to mess about a bit between headphone and speaker output - but neither is particularly high powered so aslong as you get the output volume levels into the speakers right shouldn't have any issues with "double amping".
What monitor do you have BTW? a small number have reasonably decent DACs and low enough power output sticking speakers in there isn't a problem in terms of distortion, etc. and again more removed from noise generated within the PC itself.