Suggestion: another flash or softbox/umbrella

Hello Friends,

As title says, should I purchase another flash or softbox or umbrella for my existing SB600?

To give more details, I am hobbyist taking pictures for fun and learning with interest in portrait, landscape, actions. I already have one SB600 flash for Nikon D90 and few lenses. I have few bucks available to use starting next month. Usually for portraits I go to the club during model photoshoot event and there I don't get enough opportunity of changing the light setting myself. Just use your camera and shoot , I have got few nice portraits of models. Not sure, where to start to learn lighting and setup (I do have understanding of bounce/reflector etc) using either multiple flash or using flash modifier like Softbox/umbrella.

While browsing on ebay, found one 24"x24" softbox.

http://www.ebay.com/itm/Softbox-SpeedLight-Flash-60cm-24-Flash-Softbox-/110566038425?pt=LH_DefaultDomain_0&hash=item19be3fdb99

Looking for few suggestions,
Ray