If you are an audiophile with hearing sensitive to delicate differences, then go for the dedicated sound card. But the Realtek ALC887 8-Channel High Def sound chip on the motherboard is really quite good. And if you are a gamer, you'll be too busy to notice the differences even if you were an audiophile. And, unless the dedicated sound card has its own processor, you will still loose a few clock cycles even with the dedicated card.
I've used both the onboard sound with my motherboard and a dedicated Asus Xonar card which received good reviews. I can't tell a difference between the two...sound cards seem to be going the way of the dodo for mainstream gaming, frankly.
^^ Again, depends on the speakers/headphones used. Getting a $1000 soundcard won't do squat to a $20 set of speakers.
In just the past 4 years, I've gone through 4 soundcards [ExtremeGamer, Razer Barracuda, ASUS Xonar D2, ASUS Xonar Xense] and about 6 different headsets. There were significant differences depending on the exact setup.