According to Moore's Law the NX should be roughly 4x as powerful as the Wii U (and more efficient, of course). That puts it in the power range of PS4 and Xbox One: 8GB RAM, 8-core CPU, roughly 1.5 TFLOPS GPU.
Personally, I think it will be on par with PS4. In some regards it will be a little weaker (CPU), in others it will be stronger (GPU). Nintendo will make the system as power efficient as possible and maximize real world performance and balance of components instead of on-paper performance as that's what they always do.
It will have Flash storage though (probably 128GB) and that will make gaming forums explode in rage.
Edit: And it will cost $249-299, bundled game included. The Wii U was $349 at launch but that had a very expensive controller and Nintendo won't pull that move again.
