Obviously a dedicated sound card would be a good choice but onboard sound can prove of excellent quality, particularly those from Realtek and C-Media.
In fact the onboard soundchips from these comapnies can also be acquired in PCI card form and at remarkably keen prices.
I used C-Media 8738 onboard sound for four years with a former motherboard until I was given a SoundBlaster Live! sound card.
The difference in the sound quality was remarkably marginal, with the Creative card having a little more "body" - since that time C-Media has released even better onboard sound products.
There are, of course, specifiations laid down for AC '97 onboard sound chipsets. See, for instance, these links: