What are basic hardware requirements of building a web server with database?
You haven't provided enough information to properly answer that question. There are a couple things that will greatly influence the server requirements. First it will depend on what technology platform you are going to use, eg. what language, type of database, web server, etc. For example if you are going to run PHP and Mysql then Linux would be a good OS, if you are running ASP.NET and SQL Server then you would want to run Windows.
As for RAM, Harddisk and CPU, this will depend on how big your site is, and the volume of traffic you expect it to receive. You also need to consider what kind of reliability you need. If you need you site to be very reliable then you need to start thinking about things like redundant hard drives or even multiple redundant servers.
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Basic Hardware Requirements
The basic hardware requirments for building a web server with database includes a web server machine, a database storage machine, a dedicated firewall for security.