Hello, I'm still in the deciding stage between a 12" pbook and ibook. I keep hearing I should buy more memory immediately (I'll mainly use it for photoshop). What I don't understand is this - does buying less-than-max amount of memory preclude you from buying the catch-up amount later (without having wasted money)? Or can you add little by little, and not have to ditch what you already bought? And is the answer different for the powebook vs ibook? And whether I buy it from Apple or elsewhere? I only want to spend my money once, but maxing out memory is expensive! Hope this question makes sense, Thanks!