I am in the same boat as you, in a way: I have held on to my D700 despite its coating of dust (not really dusty, but it elucidates my point about how little it gets used) because I think that it has more value when I do use it then getting rid of it altogether. Then again, maybe I'm best just parting with it since I don't use it...
I love the X100, and the reason I didn't jump on the XP1 immediately was the size of the lenses. They are, frankly, a bit large. The X100 fits easily in a coat pocket, whereas the XP1 seems too big for that. I am happy to wait for now and shoot my M6 and X100 in tandem. Perhaps the XP2 will grab me more than the XP1, who knows...?
I'd say think about what you use your Nikon kit for. If you do work that you actually get paid for with it, then perhaps it's worth keeping until you're sure you can accomplish the same work with the XP1. If that's the case, sell it. On the other hand, if you're only in it for the hobby, and you don't like shooting with the Nikon kit, then why haven't you sold it already?!
Hope it works out,
Trevor