Onboard sound is more fine than fine these days, I have onboard connected to my surround sound reciever, and get pretty good 5.1, but I have never had a seperate sound card on decent speakers to hear it.
As for video, onboard is all you will ever need for surfing the internet, but isn't much use for the modern day 3d games, but you don't need to spend $300 to play the odd game, there are $60 carsd that can play games with good eye candy, with a good frame rate.