If you're just going to do a digital signal out (Toslink, coax, etc.), there will be no difference between the mobo and consumer sound cards. The only point to getting a consumer sound card is if you want surround sound, or want a reasonably competent analog out (not digital out). I hear absolutely no noise on the Realtek 899 implementation on my Gigabyte X58A-UD3R, have tried a few Xonar cards, and have concluded that I hear no difference at all going from the optical S/PDIF (toslink) out from the sound card to the mobo toslink, just like everyone says there's no difference. I did hear an improvement going to 75 ohm coax, and the X58A-UD3R supports both Toslink and coax, so I removed the sound card from my desktop since it's now completely unneeded.
There are specialized pro cards (supposedly less jitter (doubtful) among other benefits such as AES/EBU output), but those begin at MSRP $650+. I would not even consider touching those if your source isn't in $5k+ territory.
I'll agree there is a big difference between notebook and desktop sound implementations, but the difference I hear between a good Realtek component and a $100-200 sound card is marginal at best.