See
this thread on Tire size calculators. I'd say the typical drop (remember, I'm no expert) is 1.5in to 2in. And if you do it, you want to do both springs and shocks together... and if I were doing it, I would want to have adjustable height and dampening.
The other alternative is called air ride suspension (I'm sure you've seen those). Comes with a compressor and can be lowered nearly to the ground for shows, and raised to stock height for normal driving, etc.