Hi. I think most people would suggest that you purchase additional memory from somewhere other than apple.com. I would also think that most people would suggest purchasing good quality memory, which crucial is. When I bought my PB, I decided to replace the 2 256MB memory SODIMMS with crucial 512MB SODIMMS from newegg.com. A lot of people will also move from the 2 256MB SODIMMS to 1 512MB SODIMM when they order the PB from apple.com. (I didn't do this, as I already has the 2 512MB SODIMMS when I got my PB). I don't see any benefit between the 15 and the 17 PB, other than screen size. I think you need to determine what you want to use the PB for, and pick based on that.
Kimo