I should also add, to the disclaimer in part 1. above, that you should compare your springs to the ones I run. Softer springs with a similar or lower drop, could mean your car is more prone to rubbing and understeer.
I run these:
http://www.crucialracing.com/products/springs.php
All the specs are listed, there.
Pic of my car with them installed:
http://www.crucialracing.com/images/springscodyscar.jpg