Most onboard sound cards have VERY poor frequency response, which is why many use after-market cards.
I would agree that was true in the past, but with the AMDs release of the 690g chipset they substantially raised the bar for onboard sound. Since then Nvidea and even Intel have released onboard sound chipsets of good quality.
I ran my system through my onboard sound for a little while (690g chipset) and it sounded darn good. I did notice a slightly higher noise floor (a very slight hiss in silent spots between tracks).
