Depends on what you want to do.
You could hook up the motherboard to the receiver via analogue cables and get 5.1 in games that way. But if you want dolby digital headphone (which is pretty overrated if you ask me), then yes, you would need some kind of sound card.
Edit: actually, I seem to recall some receivers can also encode for dolby digital headphone, so you could theoretically hook up your headphones to the receiver. I'm not sure about this though, and even if it's possible, your receiver might not support it.
Edit 2: or obviously, what GPU are you getting? Almost any new one will support HDMI out, which is by far the best solution.
Edit 3:
These are the type of cables you need to hook up your motherboard to your receiver to get surround sound in games without requiring a soundcard.
