many on-board sound cards including many VIA chipsets (that use realtek audio) don't have an equal amount of DB gain across the front/certer/rear jacks. this will affect the performance of products like this:http://www.overclockersclub.com/reviews/zalmanzmrs6f/
where an equal amount of gain is important because those headphones don't have an external amp. your rear speakers will also always sound slightly quieter than the fronts.
you still can't go wrong spending a few extra bucks on an audigy, even though there hasn't been much excitement with them lately.