[blue00gt]

Specific rate springs have a linear force vs. displacement relation while progressive rate springs get stiffer the more they are compressed. For example, if you have a specific rate spring with a 500 lb/in spring rate, 500 lbs of force on it will compress it 1", 1000 lbs of force will compress it 2", and so on in a linear fashion. With progressive rate since the spring rate increases as it compresses, it will take more than twice as much force to get twice the displacement. What this menas in terms of how the car performs is that with progressive rate you can have a more plush ride while you are just driving along, but the springs get stiffer the more you push the car and load up the suspension. For the best handling specific rate is preferred, but ride quality suffers slightly compared to progressives.
It depends on the exact year and model of the Mustang, but stock late models come with about 420 lb/in up front and 200lb/in out back. Performance springs for improved handling usually start at about 500 lb/in up front and 200 lb/in out back on the softer end of the spectrum; and go up to about 1000 lb/in up front and 300 lb/in out back for maximum performance. If you want handling springs and don't want to invest in coilovers, I would recommend either H&R or Eibach.