Thanks for the responses...
I should have mentioned I'm a student (or will be shortly) - does that make a difference on recommended place of purchase?
On the DVD burner thing - I don't understand the technology that well - the dual layer increases storage capacity, correct? So I'm probably better off waiting for that to appear, rather than adding it with Apple? That's probably the only real reason I wanted to go through Apple.
Good 3rd party RAM is as good as apple, and cheaper as well, correct? (ie Crucial)