The Nforce2 onboard sound has pretty decent quality from what I've read. I don't think you'll notice a huge sound difference in your everyday stuff if you use the digital outs.
The main difference is the support for input/output device. The Audigy's will have a lot more in terms of different input and output ports than the onboard Nforce2 sound. In addition, the Audigy 2 has added some new features like DVD-Audio support. If there's a specific feature on the Audigy's specsheet you need, go with that. If you just want good quality sound for everyday stuff, the Nforce2 (thus the ASUS A7N8X) will do well.