With shorter springs, you can't have the same smooth ride you get from stock springs. Stock springs are longer and can travel a greater distance upon impact, so they can be softer without producing coil bind (coil bind is where the spring supresses completely, binding the spring completely closed). The softer spring allows the car to glide over imperfections in the road, where the stiffer spring transfers a lot more energy to the vehicle upon impacts.
The balance here is to keep the spring as long as possible while getting the look you perfer. A drop of 1 - 1 1/2 inches will give the car a much better look, but won't sacrifice too much driveability. If you don't mind a stiffer ride, you can go as much as a 2 - 3 inch drop. Be be aware of curb hights, etc. when making these changes, because you may find yourself cracking a bumper cover if you aren't used to parking the lower vehicle.
Another thing to keep in mind when purchasing your lowering springs or racing suspension is the height of any aftermarket body kits you intend to install. These body kits usually give the vehicle less ground clearance, and adding that with shorter springs can have an undesirable effect. Also, any undercar lighting may be in danger if the vehicle becomes too low. Take these things into consideration when choosing your lowering springs, and you should end up with the results to are looking for.