As an open source product, Cyrus runs on any platform you can build it on. You can bet that Cyrus is more easily built on systems that are somewhat commonly used in the industry. Forget about using that ancient PDP-11 you found in the basement.
Because of its resource demands, choose a machine that can:
Support the amount of memory your Cyrus user base requires
Reliably support the fastest network media available on your network
Support robust disk storage and fast I/O