For the modeler you need Java JDK 1.5+, .Net 3.5 and MS Windows.
A trivial server setup can run on the built-in mendix web- and database server, and only requires Java JRE 1.5+. A server wil run under both MS Windows and Unix based OS-es.
For a server with optimal production performance a server with PostgreSQL 8.2+ or MS SQL Server 2005+, and Apache or IIS is advised.
Hardware requirements depend on the complexity of the application, the amount of data to process and the number of concurrent users. In my experience, a simple application will perform even on a three-year old laptop.