My 64 Bonneville has fantastic handling with the stock (soft) springs. The sway bar is bigger and I use Bilstine shocks among other modifications.
My area (Western Pa) is all bends and hills. I've had my car up to 145 mph and its as smooth as silk.
I think air springs are a waste of money. Spend it on shocks, tires and a Lee steering box.
I use 225/70-15 Firestone Firehawk PV-41 (police) tires.
http://ssl.delti.com/cgi-bin/details...ountry=US&shop
My car handles like its on a rail and rides very nicely.