You have alot of choices depending on how low you wanna go and how you want the car to handle, etc.. Some springs are good for looks, but will not cut it for handling purposes. There really are alot of choices for you. However, if you want 20's, your probably not into making the car it's best, or racing it, right? So your basically going for looks here? In that case, I would go with cutting the front springs maybe a half coil to a full coil depending on how low you want it, and using some Moog rear springs which are really cheap from Summit.
That set up will get you to about stock SS height (depending on how much you cut the fronts), AND it will cost you practically nothing.
If your looking for handling, or decide to not go with 20's, I would go with Hotchkis springs. On a 95 or 96 SS it will lower it by 1 1/4 inches up front, and 1 inch in the rear. So on your caprice it will lower it quite a bit more than that due to the taller caprice springs. For the money (about 220 bucks from No Limit's), and for what these springs do, they are probably the best set up for a b-body. I have them on my car, and am extremely pleased with them. However, I'm into making the car handle like it's on rails so it all depends on what you want.