In terms of software - PHP 5.1.3 or greater, PostgreSQL 8.1 or later (or MySQL 5.0.32 or later if you _have_ to but it's not recommended), any version of Apache and most linuxes are fine. People have had success on MacOS and Solaris, but that's not officially supported. Much less success on Windows, Mahara 1.1 will address this somewhat but it still won't be officially supported.
Hope that helps - to give better information I'd need some numbers about number of users you expect over what time period, etc.
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