I wouldn't lower it much more than 2-2.5" if you're going to drive it regularly on the street. For best results, you're going to want to purchase a high quality set of coil overs with offset top mounts. I suggest the ones made by Fortune Auto. You also need to get a set of modified 3rd members for the front suspension from Loren (his username is DMSentra). This will keep the front shocks from binding up against the 3rds under compression. You can also purchase a set of upper control arms from Loren that will give you more negative camber up front.
Keep in mind that when you go very low, you're going to put the CV axles at a bad angle which will cause driveline vibration and severely shorten the life of the axles. My car is lowered about 2" (I have about a 10mm gap between the tire and the fender) and I have a decent bit of shaking going on when I'm on throttle.
Our car's suspension isn't really well suited to being "slammed" like you see with VWs, 240s, and many other cars. There are some custom motor mounts being developed to raise the engine and transmission to allow the car to be lowered more, but I don't know how long it'll be before (or even if) they'll be sold to the public.
