Only true Audiophiles can discern the difference in onboard sound and a soundcard. I prefer a sound card just so my pc has more time to due manly pc duties/chores. It will slow you down some in a game, and hardcore gamers hate this, thats why Creative has a niche in the soundcard market. With Asus and others (DFI) using the PCIe 1x slot (sometimes 4x) slot for Add in daughter cards, the noise and interference associated with onboard sound is pretty much eliminated (unless they leave the audio codec on the board, but thats a whole new can of worms) So in a word, yes, it will help your gaming. Enough to make a difference, depends on what you play. IF you play a physics, graphically intensive game that not only taxes your graphics card but also your cpu, then, unless its dual core (and even then Im not sure how much that helps) the CPU has to, while doing the previous task, switch to and also calculate the sound effects and all that, then it would slow you down. But as I digress, it all depends on what you play and what other stuff you have in your pc. Hope this helps.
-The Eagle