The realationship to watch is between spring rate and car weight (sprung weight). As the spring rate increases the natural frequency of the suspension goes up. If that frequency gets too high then the car becomes unpleasant to drive. The high frequency "jiggles" give me a headache but they cause other problems for other folks.
If the spring rate/car weight ratio is too low then the car feels like a boat.
You typically don't want to get the natural frequency of the suspension much over 1 Hz for a street car. You can go a little higher if you're willing to buy really good shock absorbers.
