If you feel you'd benefit from a soundcard, make sure you get something that plays at least 24-bit audio @ 44.1 Khz with a quality DAC. I have a Digidesign Mbox 2 and I love it. At times I could hear static and other unwanted noise when I connected straight to the audio on my motherboard but I hear nothing but crystal clear audio from the mbox 2. Also, if you haven't seen Logan's video on soundcard and audio myths, I highly recommend it. He basically explains why most soundcards are just glorified copies of the audio chipsets that already exist on our motherboards and are a complete waste of money.
As far as I know, Macbooks actually have a fairly nice audio chipset that is very nicely secluded from hardware noise. Unless the heat of the Macbook getting too hot killed something in it, I don't believe you should NEED to get it unless you REALLY want it. I only take this stance because from what I've heard from audio recorded onto Macbooks and played from Macbooks has sounded phenomenal to me, much better than most on-board solutions that I've encountered and internal sound cards.
You do not need a sound card, after testing with several headphones I realized that macbooks have a pretty decent amp, it's actually better than what you would get if you buy that soundcard you are looking into.