IIRC, for the most part, onboard sound is exactly the same as your average 50 dollar card. The chip is just embedded in the motherboard instead of in an expansion card. At least thats how it is with both of the MSI boards I have. I'm not too sure on the Abit one. They are getting better in terms of sound quality, but generally, I would go with a cheap Soundblaster Live! Value, or equivalent over onboard sound. I've had this one for about... whenever it was when they first came out.
As for bus speeds, throw as much stuff as you want on there. The thing that takes up the most bandwidth on the bus would be the video card, and only during heavy use. The sound card and NIC are inconsequential. Even under heavy load, they take up next to nothing. Its why they made CNR (Communication Network Riser) and AMR (Audio/Modem Riser) slots. The little brown, quarter size ones. You can get modems and sound cards that will fit in an AMR, and NICs that fit in a CNR.