How much ram do you need for 100M hits per day?
My answer to How much ram do you need for 100M hits per day? on Quora
That entirely depends on what you are running your site on. If it’s static HTML pages, nginx will work fine on hardly any RAM at all (though you would probably be better off serving the entire site out of S3 and not using a VPS at all).
For dynamic pages, the only right answer is to run a load test and see what happens. For that kind of traffic you’ll probably need more than one server if you’re serving dynamic pages though.