I have a 92 Si which is the exact suspension geo you are running with just a little more weight. 750f/450r with stock front sway and 24mm adj rear has been extremely flexible and stable for me. Springs are cheap... shocks are expensive. Those Koni's are adjustable and will give you the simple play settings you need to swap springs anytime you want. Most succesful EG suspensions (both rr'ers and auto'xers) have greater front spring rate than rear - time and time again. For your reference, DMS road rally tarmac spring rates are 700/400 with stock front and small rear sway... which allows for VERY stable car. Go stiffer as you feel more comfrontable, as you change sways, tires and your driving style. For instance, I've been running my setup for 3 years and I've been VERY happy. Now that I've running hoosiers, I plan on going up +50 lbs all the way around for testing purposes at a TT. Too stiff? No big loss, I take them off and be done with it.

Also, try redshiftmotorsports.com for GC/Koni parts - tell Chris 'mickey' sent you.