I don't understand if you're looking for 2x1GB or 2x2GB, but IIRC won't the Macbook will top out at 3.3GB?
However, I don't know the answer about what's fastest but I suspect you'll never notice any difference with any brand named memory (i.e., Crucial, Mushkin, etc.)
...if even that much, since the bus speed is probably the main limiting factor.
You are right, it is the amount of RAM, not its speed, that will most significantly impact the end-user experience. 3 GB of standard RAM will be much faster than 2 GB of "fast RAM" for most applications.
I think a lot of people go for the branded stuff because the companies that sell them usually have better i) reputations, ii) customer service and satisfaction, and iii) warranty and return policies.