FreeOrion does not yet have official system requirements, and what it currently needs is highly likely to fluctuate throughout the development.
As a rough guideline, the game uses OpenGL, meaning you should have at least a 3d accellerator card. If you have experience running it on a particular system and thought it was a bit slow, please write your requirements here so we can consolidate them into some guideline of reasonable specifications.
Operating system you will currently need either Linux or Windows. A MacOSX port is underway; ports to other systems are plausible but require developers from them to put in the time to see it ported.