It's important to remember that AC97 sound is software driven. If Creative's Audigy drivers are written correctly (hardly a given with anything made by Creative, their drivers have recently been pretty poor) the Audigy should dominate any AC97 based software audio solution with regards to performance.
Sound quality is very subjective and depends a LOT on proper board design. Onboard sound audio quality varies tremendously, even between boards using the same AC97 codec. (Realtek, CMedia, and Analog Devices are the manufacturers of the majority of AC97 onboard sound chips.) It's generally a safe assumption that the sound quality on a good standalone sound card is going to be better than the average onboard sound solution. Typically onboard sound solutions deliver acceptable sound quality. In my experience there are two major failings with most onboard sound solutions:
1. Stereo separation, which is basically the amount of bleedover between the left and right channels, is typically very poor compared to standalone sound cards.
2. MIDI wavetable implementations on some onboard solutions is downright terrible. (Try the "trumpet" sound on some onboard solutions, it's sometimes so bad it's almost funny.) Truthfully though, not many of us deal with MIDI files an aweful lot anymore so this isn't really a huge deal.