Another good thing about softsynths -- you will learn an insane amount about synthesis and sound design, because you'll be exposed to many more different types of synthesis (not just subtractive, but FM, Additive, weird variations like HCM etc) than you would even if you had a couple of dozen HW synths.
The softsynths feedingear mentions are all fantastic plugins and worthy of your collection, but IMO they tend to isolate you more from sound design than something like Zebra2 or KarmaFX which are modular in nature and give you an enormous amount of visibility of the nuts and bolts of a sounds. Also if you're interested in books on the nuts&bolts stuff, this guy has written a few:
http://simoncann.com/ .. You can get one of his books in pdf format free at this link:
http://noisesculpture.com/how-to-make-a-noise