What do you need a more expensive camera for? If you think spending $1800 on a new camera will give you better pictures than your D40x, you're pretty way off. At the least I'd say invest that $1800 in good quality lenses. At the most I'd say get a used D80 or D200 (if you think you really need a prosumer body) for the built in autofocus motor and get better glass. The new Nikon AF-S 16-85mm f/3.5-5.6 VRII, although on the expensive size, has superb image quality and build for a consumer lens.
PS: Your title is wrong. >$1800 means greater than $1800.
