Many different spring rates available to accommodate different rider weights, load weights and riding styles/events. For snowbiking, most recommend the stiffer the better. The super stiff ones are a bit harder to find, so check more than one source before just getting the stiffest ones that one retailer carries or can order. Search around a bit. Best to revalve to match the springs but not absolutely necessary, but note that the valving on an FX is settup up to match the softer FX strings compared to the F motocrosser.
When I did my KTM EXC, I did both springs and valves because I was coming from the softest end of a stock setup. The FX is only one step down from a motocrosser. Maybe do the springs and then determine of you really want to dig in and do valves as well?