mmmm to answer that question, start from how many users will you have? This is modified by how many courses are you going to have? This is modified by how many teachers are you go to have running their courses via Moodle? In short... how long is a piece of string?
General rule of thumb. If you have a lot of users, then a database server is more appropriate as long as it has a bucket full of RAM, as much RAM as you can give it. This Moodle Doc on performance issues might give you a lead.
I know one site I was connected to had the moodledata folder and database on a separate server (or maybe on separate servers), and AFAIK, never had performance issues. I never found out how much disc space nor RAM they had either. I did ask but didn't get a response, and I object to Secret Squirrel so never bothered again.