how much capacity of a server is required to run a medium class website?

Answer 1

Well the answer to that depends on what you call a MEDIUM class website.

The real answer depends on how many hits per day you expect. Then besides the server there are bandwidth issues that must be considered.

The server itself only has to store the website contents so its hard drive capacity is normally not the issue. Now if you are talking an active (interactive) site with a database users signup etc that changes. The memory requirements, number of network cards, etc all change depending on how many users are connected at one time and whether the bandwidth is available for all the users that are connected.

I know that didn't answer your question but you need to give some more information on what you are asking.
Since webservers are "scalable" you can grow as needed in most cases and not have to be overly concerned with what you start out with.

Answer 2

Not a lot is needed unless you plan to have a huge amount of traffic. I have a database driven website with thousands of articles, but very few visitors (as it is new). My disk space usage is so small it reads 0% of the amount my server gives me, as does my bandwidth lol

Even the smallest of web server packages online these days will suit most people, such as the smallest one from hostgator.com