Regardless of how much better an add-in sound card sounds or performs compared to on-board, you need to weigh the price premium most board makers charge. Chances are you'll switch to a new motherboard before you want a new sound card, and you can just port your sound card to your new rig.
I've typically always used a Creative card though because EAX2+ is just awesome in games that support it. The closest I came to using on-board was with my A7N8X-Deluxe and its excellent SoundStorm solution. I'm not sure if there's any current on-board sound that offer the same quality, but some of the boards I looked at this time around were charging $300+ for bells and whistles I didn't really want to spend that much more for.
There's been a few boards announced that are going to have X-fi on-board, which is probably the worst solution since the price premium is pretty visible. It also seems onboard X-fi is the main reason for their delay since Vista support is still a bit rough.