Well one advantage of your onboard compared to Creative's offerings is that you don't get the distortion that comes from resampling 44.1KHz music to 48KHz.
However, analog noise level is still not comparable to a real PCI offering.
The real strength of your nForce2 is using the digital out, and having the option of hardware Dolby Digital Live, which assuming you have a decoder, will be cleaner than any analog source since the resampling rate will be thousands of times higher and not prone to any errors(at least theoretically), as well as a very low noise floor. Assuming you have a decoder of course