I’m not sure the analogy works, so I’m going to avoid the analogy altogether.
Your computer wants to use a great deal of RAM, probably more than you have installed. It makes up for this by treating some of your hard disk as ersatz RAM,and keeping only the most important and frequently used things in actual RAM. When it needs to use something that isn’t in actual RAM, it needs to copy it from ersatz RAM to actual RAM before it can use it.
So the optimal amount of RAM depends on your usage patterns. If you tend to use one program at a time, you need less than if you tend to flit back and forth from Mail to Safari to Word (or Pages) to Adium to Terminal. See, if you have all of those open, but you’re only using Safari, all the others get copied out to ersatz RAM, and since you’re not actively using them, you don’t notice the lag that follows switching to another program. On the other hand, if you are in Safari and switch to Mail, the computer needs to make sure that all the important bits of Mail are in real RAM, not ersatz RAM, and that causes a delay if they aren’t.
As a practical rule of thumb, it’s a good idea to get as much RAM as your computer can handle, because it’s the cheapest way to boost performance. The advice, though, is to do this cheaply: if you ask Apple to do it before they ship you the computer, they’ll charge significantly more than it would cost to do it yourself.
If you want to ask more specific questions when purchase time approaches, feel free, of course, but that’s the general theory.
Oh, and if you talk to other techical people about this, the correct term for “ersatz RAM” is “virtual memory” or “swap space,” but both those terms are fairly opaque for non-cognoscenti.