Here's my take on it.
Yes, onboard sound can take more CPU cycles. The more channels you use, the more CPU cycles it eats up so if you tend to do stereo sound only (like myself, wearing headphones at LAN parties) then there isn't much performance hit. If you plan to do a 5.1 or 7.1 speaker setup to immerse yourself in sound, then the CPU hit will be higher. However, that extra $50-100 for an Audigy can go towards a faster CPU or faster video card - think of it that way.
As for sound QUALITY... that one really depends on the motherboard. Some boards sound perfectly fine, others have audible artifacts whether it be hiss, crackling or picking up system bus noises (such as HDD or mouse activity). Yes those audible artifacts do exist. No, they do not exist on all onboard audio. Also, they are not always audible depending on your sound setup. For instance, using cheap speakers with my Abit IS7 a few years ago sounded fine... as fine as cheap speakers can sound. Going to a stereo receiver and shielded bookshelf speakers and I heard some crackling and such. My first LAN party with it and using decent enclosed headphones, and the crackling was unbearable. The sound on my MSI PT880 board makes a buzzing in conjunction with mouse/HDD activity, but again most noticeable on good headphones. The onboard sound on my Biostar Tforce boards that I've been using in my LAN gaming rig sounds just fine.
Conclusion? Use onboard sound. If it is fine to your ears, great! If not, then spend more money on a sound card.