manufactures and others claim that because without a sound card, the processor has to do all the decoding and all that great stuff because the info it sent to the chipset and then sent to the processor and back. however, with a sound card, the chip in the sound card will do all the work.. same thought as a video card, but video cards are more dramatic because programing for the cpu and gpu is a whole another story.. maybe until Intel Larrabee or AMD Fusion in the next year or two.
anyway, now a days, its basically moot because the of the dual core, quad core processor and $10 GB RAM and the much improved quality of onboard sound. only reason to get a sound card is if you can or care to tell the difference in audio quality or want more outputs/inputs.
lastly, if anyone tells you the killer LAN card works, tell them Macs can never get viruses.