I have a Xonar ST and actually can't tell the difference between the digital out on it and my on-board digital out. Jitter would be the main deciding factor on weather it is a bottleneck or not. Some on-board audio is full of jitter and some audio cards are also full of jitter. In this case, the Xonar ST should give you an improvement on jitter but weather you can hear it or not is up to you, let alone the price of the ST for mere jitter upgrade may not be such a great idea. Aside from Jitter, you may experience a dropout every once in a while with on-board audio when starting a CPU heave task such as a game or rendering application while listening to music, soundcard may or may not fix that. If Jitter is of concern you can get a PCI or PCIe soundcard in the 20~50 dollar range from a reputable manufacturer and there should be improvement. A soundcard could also provide signal processing like Dolby Headphone or ProLogic II if you wanted that as well.
Slightly related, I recently stopped using my ST in favor of the on-board digital out to my rig and am quite happy with it. There was nothing wrong with my ST but it blocked one of my PCIe slots which is now home to another video card.
Edited by sulvaat - 4/11/11 at 7:24am
