For more on yams and sweet potatoes, see my update to Why I'll Never Be An Atkinite.
A yam is not technically a sweet potato, although those terms are used interchangeably in the US. A true yam isn't bright orange, moist or sweet. It's pale, dry and starchy, more like a white potato.
Here's a nice table that lists the differences:
What is the Difference Between a Sweet potato and a Yam?
So, my analysis is actually for a sweet potato. And my picture is technically of a sweet potato. I'll come clean, I've never tasted a true yam. Have you?
Er, ah. Make sure you poke a few vent holes in your spud before you cook it.