The current D2x sensor is 4288 x 2848 pixels in a 23.7mm x 15.7mm sensor. This equates to 12.2 mp at a density of 181 pixels per mm. If Nikon were able to replicate that density on a 36mm x 24mm sensor (and I can see no reason why that is not technically possible, expensive but possible) you would have a 28.3 mp sensor (181 x 36 x 181 x 24).
The fun begins when you start playing with numbers that Nikon themselves have admitted are possible in the future. They have stated that they think they can get 24mp out of the DX sensor, which equates to 253 pixels per mm. Turn that into a FF sensor and you get 55.3 mp
I think my 2.8g pentium may struggle if I chuck a 55mp image at it
