Basic hosting system requirements guide?
 

Hi there. Is there any sort of guideline or resource as to recommended hosting system requirements?

When I go out to buy a program or an operating system, the product has some system requirements.

I realize that vBulletin is different, but surely after all the years of running there can be some basic guidelines on server recommendations?

For example:
vBulletin 3.8x
100-250 current users
Recommended minimum X Server hardware / software configuration

vBulletin 3.8
250-500 current users
Recommended minimum X Server hardware / software configuration

Or even an article on choosing server with some guidelines on processor speed, or how important drive speed or amount of ram is needed.

Maybe there is a resource already that I'm just not finding.

I realize there are so many scenarios and variables, but right now I'd be happy just to know some basic ballparks and guidelines.