Basically, non-sports will drop 40mm comfortably. Then if you want to drop any more you need to modify something to allow for clearance (I think it is removing the rear mud guards because they'll rub against the rear wheels), then you can drop to 60mm. But the ride is meant to be quite harsh at 60mm, so I recommend 40mm for just springs. Plus the Eibach Sportlines will only lower by up to 40mm I think, and those are meant to be the good ones.
By the way, a halfords guy told me that stuff about the rear mud guards, I'm not sure if it's correct since he seemed quite edgy himself! :P