In Nikon’s own words, the D40x can be thought of as a D40 outer with a D80 inner. The body and controls are identical to the D40 and as far as we understand, the sensor and shutter mechanism are taken directly from the higher-end D80. By borrowing the D80’s sensor and shutter, the D40x also inherits a broader ISO range starting at 100 rather than 200 ISO, along with an increase in continuous shooting from 2.5 fps to 3 fps. Sadly it also means the D40’s impressive 1/500 flash sync speed has now become a somewhat average 1/200 on the D40x.
Despite sharing the same sensor as the D80 though, the images aren’t identical. The D40x applies the D40’s image processing to deliver brighter and sharper, more consumer-friendly JPEGs than the D80 by default (although you can of course tweak the settings). The D40x’s RAW files are also slightly different to the D80s thanks to its different image processor, so you'll additionally need up-to-date software to process them. Read the whole review here