Well IMO almost all built-in soundcards sucks. Why? Because they are made of cheap components, uses CPU power to create sounds and the general quality of them are often bad.
I know of only 2 kinda good built in soundsolutions. The first one i Soundstorm that was about the same level as the Audigy 1. The second of is the SB HD Live! built into a few MSI m/b afaik.
The one used on Asus NF4 series is not good enough for me atleast. Thats why I got an X-Fi ExtremeMusic...
And even if the onboard sound looks flashy on the paper it rarely lives up to it. But I guess it is mostly about demands and what you have to compare with. If you think the built in is sufficient, don't put money into a separate one.