No you need good processor power (quads) and feck loads of RAM!

But also you are better off buying Renderman and extra ram for the money of a new machine - RMan wont give you any bullshit. Mental Ray calculates every model into tris before it even squeaks, even then it dies of prematurely ejaculated calculation time and throws you up an error, saying 'oops sorry got any tissues' wheres as RMan doesnt get bogged down with that type of thing.

Cheers
Jay