20kg/mm fronts and 25kg/mm rears. Anything less and you are a bitch.
But seriously it all depends on the spring itself. If you are getting BCs get the Swift springs in something like a 8k front 10k rear and you should be pretty good. The reason I say to get Swifts is because BC springs are not very good and a quality spring will allow you to go up in stiffness while maintaining a decent ride.
I run 700lb/in front and 650lb/in rear Hypercos and the car rides smoother than it did with 8k/10k BC springs.