How do I know if Im ready for FF?

I have a D90 that I've used for 3 years to shoot everything from sports (college football and HS soccer), evening events, landscape, street photography, and my first wedding. I love my d90 but it is limiting in certain situations.

My shooting preferences are really sports, low light events, and street photography at night. I wouldn't mind doing more weddings as well but man that was a lot of work. The bride loved the pics. My biggest issues came at the reception as it got darker which part of the reason Im at this crossroads. I think a D700 would benefit me due to the better ISO capabilities but Im not sure if that's a good enough reason.

My current gear includes: D90, SB600, Manfrotto 55xprob, Nikon 70-200VRII, Nikon 50 1.4G, 18-105 kit, and a Tokina 11-16.

How steep is the learning curve going from a d90 to d700? Is better ISO a good enough reason to go FF? Why do you shoot FF as opposed to crop?