I'm currently using the built in sound on my Asus Extreme III, it is fine, though I did buy a Asus Xonar sound card as well, just didn't put it in yet as wanted to test things out with the built in.

Not all on board sound cards tax the CPU the same, I'm sure the lower end boards are taking what they can from the CPU, some of the higher ones I looked at pretty much just include the sound card on the board. With the shrinking of everything and the fact that sound needs just haven't scaled like video sound has kind of just become this low cost thing we expect to have.

That all said though I did buy a sound card as I figured they are so cheap (even cheaper with a combo deal I got when buying parts). why not offload a bit.

There have been some reviews of on board sound, and some of the MBs have done quite well in that regard.

I'm skeptical whether I could tell the difference in quality of sound from what I am using now(built in on my Asus extreme III and the card I'm going to pop in this week (Asus Xonar). I'll let ya know.