You are probably smart to do it now, since it appears you don't have a lot invested in high end glass and much of what you have is in need of a refresh anyway. Timing is probably good. I'm guessing you can probably net enough to cover about half the cost of a D800 with a kit lens.
I never try to talk anyone out of switching brands, bodies or lenses. If you are dissatisfied with what you have and feel something else would be better, it's unlikely you'll ever be happy with a compromise. Rather than be unhappy with a 5DIII and always thinking you should have gone with a Nikon, its probably better to just cut your losses and switch.
You should recognize though, that it is going to cost you a lot more than the $500 difference between the D800 and the 5D III to replace what you have.
As far as selling goes, I would recommend that you wait a week and watch what identical equipment is selling for on eBay (not what people are asking, but what they are actually selling it for). That way you'll have a better idea about how to price your equipment.