Space is not too much, phpBB works well I agree.
You need roughly 10 megs of space initially for the database and it's first few hundred posts/users, the scripts that run the forum itself are very small.
PHP and MySQL are required.
If you're running a relatively small board, the older versions of Ikonboard use Perl and a flatfile database to solve the problem of not having PHP or MySQL, if that is an issue, but it's a bit slower.
http://phpbb.sourceforge.net
http://www.ikonboard.com