Which x2 teleconverter with Nikon d40x?

Can anyone suggest a reasonably priced x2 teleconverter which is compatible with the Nikon d40x?

The choice is basically limited to the Kenko Pro 300 DGX teleconverter.

The DGX model is the only one that will transmit proper lens setting information to the camera.

Generally teleconverters will degrade the images to too far of an effect you will get blurry photos.

It doesn't matter what camera you have. It's a Nikon AF-S mount, and that's all that you need to know. What is much more important to know is what lens you're using. For Nikon professional telephoto lenses, the Nikon 1.7x converter is a better compromise than the Nikon 2x. If you're using Sigma EX lenses then Sigma's 2x converter is the one to get.

For slow lenses, you will have to get the Kenko teleconverters. In terms of quality, they are acceptable, however the lenses you use with them won't have been designed with teleconverters in mind, and it's generally not advisable to use a 2x teleconverter with lenses slower than f2.8.