You're going to be pretty limited on future RAM upgrades in that thing; it's only got one slot + the 256 on board. Are you sure you don't want a Powerbook 15" instead? In any case, I'd get as much RAM as you can afford (probably the extra 512, rather than the extra 256. OS X is a memory hog. It might also be worthwhile to shop around for the RAM; Apple's markup is pretty high. However, don't skimp on good RAM. I've seen enough cheap RAM go bad, and it's miserable to troubleshoot.

I am the Doctor, and THIS... is my SPOON!