Scoop should run on any OS that Apache with mod_perl and mySQL will run on, though some take extra tweaking. It has been run on basically every flavour of Linux, some other Unixes, Mac OS X, and Windows 2000 and XP. There are currently issues with sending email (required to create new accounts) under Windows; the development team is working on sorting that out.
The computer power you need will depend directly on the number of hits your site gets and especially how big your database is. Once the database indexes get too large for available memory, performance goes all to hell and one person on dialup can DoS the site easily. That's where you either get more RAM (if practical) and/or turn on archiving.
Scoop is pretty easy to move between computers since all of the configuration is in the database, so you can start with what you have handy and move to more powerful computers as needed.