The rear of these cars do not have springs, they have Torsion bars intead. The T-bars are a PITA to replace or reindex. Which means if you want to put on stiffer front springs(which you will) you have to put on stiffer T-bars in the back. The whole rear suspension has to be dropped to change/reindex the bars.
However, you can just pop on a set of rear coilovers to stiffen the back to match the front, this involves 2 bolts per side =)
You can leave the T-bars in the back and use helper springs, or you can remove the T-bars and run just the rear coilovers.
Physically the coilovers are like the regular shock but they have threaded bodies and springs, the threaded bodies and perches make them heigth adjustable.
T-bar removal and adjustment:
http://www.tech-session.com/Paragon/Tech/944_tbar_removal.htm
Rear coilovers:
http://www.*****************.com/
I'll give you the fronts for $300 and the rear for $400.......you can come get 'em today if you want =)