Just to test, alone or with a couple of colleagues, a P-III 500 MHz with 256 MB RAM is enough, Windows or Linux or even Mac.
For production use, the plattform of choice is LAMP - Linux, Apache, MySQL, PHP. Whether it is a virtual host, dedicated host or a cluster, all depends on _your_ usage.
Moodle is much more ressource hungry than a usual static web site. So for sure, cheap mass-hosting where there are tens or even hundreds of sites on one server, is not going to work.
Once it's all up and running, don't forget the next chores: data-backups, updates, fail-over strategy, etc.