There are various flavors of onboard sound. But whatever it is, a separate PCI sound chip or chipset integrated sound, the question is that an intelligent engine in there, or does the main CPU have to do all the work.
C-Media 8738 and Creative 5880 are popular PCI sound chips with a good and intelligent engine that won't eat too much CPU time - just like an average sound card - and so is the integrated sound in SiS chipsets, and AFAIR ALi's is a good one too.
On the other hand, Intel's and VIA's chipset integrated sound engines are incredibly stupid, and need heavy CPU assistance. If it's not for the basic bleep sounds and music replay, avoid.
regards, Peter