GeoNetwork is a Java application that runs as a servlet so the Java Runtime Environment (JRE) must be installed in order to run it. You can get the JRE from the following address http://java.sun.com and you have to download the Java 5 Standard Edition (SE). GeoNetwork won’t run with Java 1.4 and Java 6 has some problems with it so we recommend to use Java 5. Being written in Java, GeoNetwork can run on any platform that supports Java, so it can run on Windows, Linux and Mac OSX. For the latter one, make sure to use version 10.4 (Tiger) or newer. Version 10.3 (Panther) has only Java 1.4 so it cannot run GeoNetwork.
Next, you need a servlet container. GeoNetwork comes with an embedded one (Jetty) which is fast and well suited for most applications. If you need a stronger one, you can install Tomcat from the Apache Software Foundation (http://tomcat.apache.org). It provides load balance, fault tolerance and other corporate needed stuff. If you work for an organization, it is probable that you already have it up and running. The tested version is 5.5 but GeoNetwork should work with all other versions.
Regarding storage, you need a Database Management System (DBMS) like Oracle, MySQL, PostgreSQL and so on. GeoNetwork comes with an embedded one (McKoi) which is used by default during installation. This DBMS can be used for small or desktop installations, where the speed is not an issue. You can use this DBMS for several thousands of metadata. If you manage more than 10.000 metadata it is better to use a professional, stand alone DBMS. In this case, using a separate DBMS also frees up some memory for the application.
GeoNetwork does not require a strong machine to run. A good performance can be obtained even with 128 Mb of RAM. The suggested amount is 512 Mb. For the hard disk space, you have to consider the space required for the application itself (about 40 Mb) and the space required for data maps, which can require 50 Gb or more. A simple disk of 250 Gb should be ok. Maybe you can choose a fast one to reduce backup time but GeoNetwork itself does not speed up on a faster disk. You also need some space for the search index which is located in web/WEB-INF/lucene. Even with a lot of metadata the index is small so usually 10-20 Mb of space is enough.