I just went through this exercise of deciding last week while choosing the components for my wife's new E1705. I was trying to decide between 2GB at 533 or 667, with the 667 option being $100 more.
The 533 is running at 4-4-4 timings, while the 667 is at 5-5-5. The 667 at those looser timings still has a non-insignificant higher bandwidth in synthetic tests, but real-world performance difference between these two memory options is absolutely negligable. I found a comparison over at THG, but I'm too lazy to look it up and post a link. 🙂
I chose to save the $100 and move up to the 7200rpm drive -- I think that will make a bigger difference.
My $0.02.
Jeff