
Some useful links & quotes follow:
BMW upgrades:
http://racetech.com/page/id/97
Correct spring rate for your bike & rider weight:
http://racetech.com/VehicleSearch
Progressive vs linear rate springs discussion:
http://www.advrider.com/forums/showthread.php?t=433139
"Does this mean any cartridge fork is better than any damping-rod fork? No, that depends on the valving and setup, including spring rate and oil level. Being a savvy suspension turner requires a knowledge of advanced engineering theory, the experience of extensive testing and art. something to remember is, the best you've ridden is the best you know"
http://racetech.com/articles/CartridgeForks.htm