even good qualitiy onboard audio on expensive motherboards is only about as good as low end cheap soundcards so really any speaker system that is self powered would work. for expensive speakers you might want a soundcard or dac upgrade though.
logitech speakers like the z506 if you want 5.1 are fine for most people. or get a 2.1 model if 5.1 is not needed. if you had a larger budget or wanted something higher quality then i suggest you give us more info as speakers range in price from under $50 to well over $1000 and come in all shapes, sizes and uses.
you already labeled which ports do what. if you bought pc speakers just connect the colors up to match.