If the onboard sound is nForce/2 soundstorm, it will probably be better than the Live! although the analog output quality of the Realtek DAC remains to be evaluated...
With that out of the way, why don't you try and see for yourself?! I mean the onboard sound and the Live! are both available to you so what's keeping you from installing the Live!, its drivers and then see what happens?
If I were you, I'd first install the Live! in a free PCI slot, install its drivers, bloatware, whatever you want to call them. Now, if anything went berserk, obviously, you keep the onboard sound. If OTOH nothing was obviously wrong, start performing some meaningful tests: Switch the output of the two (both in Windows sound properties and the actual cables) and listen to some CDs (or *good* quality compressed format audio files) that you're familiar with (Adjust the volume so that no card would be better just because it was louder, in other words their level of loudness should be about the same) and see which one is better reproducing the music (or which one feels better to your ears; pay attention to the reproduced level of detail, how natural the vocals and instruments sound, the live or artificial feel of the tracks, if the whole track is done well or if the parts hav not been composed to make a whole, etc.). Then start playing some games and see if you see higher performance in one or any weird sound problems there. Also, check to see if any of them sounds better when performing 3D Audio; How smooth the sound goes from speaker to speaker, how realistic and natural the 3D effects sound, how they affect game play (i.e. how much performance hit), etc. If you fear any compatibility or conflicts may arise, you can first disable the onboard sound and perform the tests with only one solution installed at a time. But, especially in sound quality tests, you must then try and listen carefully to find out the difference.
Well, then come back here and share your experience with us 😉