The motherboard has a decent onboard sound chip that supports surround sound. Not much point in a sound card unless you go for some good speakers or a certain headset. For example I have Sennheiser PC350 and they have huge impedance so I basically need a sound card.
But to answer your question, no sound card is needed.
All modern motherboards have an onboard audio solution - the CPU actually does most of the processing for audio. You're good to go with pretty much any new motherboard and as Wolfram23 said, there's no real benefit in a dedicated sound card unless you have high-end audio equipment.
+1 to not needing a sound card. Most games use software processing sound and with multi-core CPU's and the on-board sound chips, an extra sound card isn't needed unless you are in the same situation as Wolfram23 with a high-end headset. As for having a high-end audio system, I personally use a Denon 2808 A/V Receiver connected via HDMI to my computer with 5.1 full range speakers that you would put in your home theater attached.
The only other time I would say you need a sound card is if you buy a really high-end computer speaker 5.1 or 7.1 system that plugs into 3 or 4 3.5 mm jacks on your computer.
I can notice the difference between onboard sound and my SB Audigy 4. Not a lot, but its there, even with pretty standard 2.1 logitech speakers. But if you have never used a sound card, you wont notice anything lacking, unless you compare them side by side.
Well, a dedicated sound card is definitely better fidelity than the onboard sound, but onboard is still quite good. Leaps and bounds beyond what was offered onboard in the past, that's for sure. It'll do everything fine for now and when you have an extra $100 or so you can buy a dedicated card like a Xonar or an AuzenTech, or whatever else tickles your fancy.