I currently have a Logitech Z623 hooked up to my PC's onboard sound. Being the lazy guy I am, I haven't been really bothered about it. However for the last couple of weeks, the onboard sound has been so bad, that I have been getting very bad static coming through the speakers. Now I have been looking at buying a DAC or a Soundcard.
For the DAC I have been looking at the AudioEngine D1 DAC - http://www.pccasegear.com/index.php?main_page=product_info&cPath=211&products_id=19169
From what I had read, this DAC seems quite good. It should be able to handle the Z623 and more importantly is bypasses the on-board audio (uses instead USB).
The other option is going for a Soundcard, however might have issues fitting one in as my GPU is double slot and takes up one PCI slot.