Agavi's minimum requirement is PHP version 5.1.0 or newer. DOM, Reflection and SPL extensions are required, but these are always enabled by default, unless you're using one of these weird Linux distros made by "smart" people who think they know better.
It should be pointed out that Agavi is not a framework for beginners. To leverage the functionality and the advantages of it's structure and philosophy, a good knowledge of PHP is required, and it's highly recommended that you are familiar with concepts such as MVC, and (web) application development in general.
Also, you usually need a web server, such as Apache, unless you only want to write a console application. Agavi should work fine with all web servers that support PHP. Apache versions 1 and 2, LightTPD and Microsoft Internet Information Server have been tested and verified to be compatible with the framework and it's components, such as the routing.
If you want to install Agavi via PEAR, you do, obviously, need PEAR installed. If you'd like to grab it straight from the SVN repository, you'll need a Subversion client installed, of course. Either way, we recommend you have Phing installed, because creating a project, actions, view and all these things by hand becomes utterly annoying over time, and the command line utility uses Phing to relieve you of that burden.