The US Dynamics GP Field Team Blog (and you thought my blog name was long!) points us to suggested hardware requirements for GP 10 deployments up to 250 concurrent users on CustomerSource.
The answer to your specific question about what hardware you need is of course “it depends”. This may get you started. Also, lots of folks seem to ask for this and it’s no piece of cake to find. Enjoy.
GP 10 System Requirements
Archived post from 2007. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.