There are a lot of variables that you have left out.
Are you going to have 1 visitor a month or 1 trillion visitors a month?
Are you running dynamic content?
Php or perl or ruby content?
Is is a youtube clone?
Any mysql databases?
I mean there is a ton of stuff that will determine how much ram you need. There is no formula for determining ram. You just have to try it and see what happens. Make sure you can upgrade easily.