There was a time when I would have said RAM. But then I thought about it and realized something. The only part of a laptop you can realistically upgrade later is the RAM, so the CPU is more worthy of being upgraded if you think both are enough but you want to BTO an upgrade. In addition, the MacBook Pro comes with 1 GB of RAM. For almost anyone using this computer, that is definitely enough for whatever you'll be doing, so you can get more RAM a few years from now if you actually do need it, when it will be much cheaper.
EDIT: oh, I forgot about the hard drive. I would definitely go for the 100GB 7200 RPM HDD before the RAM as well.