If you're thinking of getting any of these cameras, there's a few things to bear in mind about them. They're all pretty much the same body, nice and small, a bright, clear 2.5" screen, three-point autofocus and a nice simple menu system for beginners. However, there's a few "issues" that they currently have.

First up, sensors. The D40 has a 6mp sensor, whilst the D40x and D60 share a 10.2mp sensor. At the end of the day, there's not really a huge difference in the images coming out of these three cameras. The thing that matters most is the LENS you put on them. A more expensive (and better quality) lens on the D40x and D60 will really do the higher resolution sensor justice and give some great images. But if you're planning on just sticking with the 18-55 kit lens, there's not much point going for the higher resolution models just for the sake of it - prints will look exactly the same, because that kit lens is holding it back.

Second of all, and probably most importantly, is the AUTOFOCUS. To talk about this, we need to look at how Nikon lenses use autofocus.
The kit lens you get with the cameras, the 18-55 zoom, is what's known as an AF-S lens. This means that it has a tiny focus motor located in the lens that moves the focussing elements around. This is shared now among all of Nikons new zoom lenses, along with their telephoto lenses, like the 200mm and 600mm, for example.
Nikon's shorter focal length fixed focal length lenses, like the excellent, and cheap, 50mm f/1.8 don't have this little motor. Therefore they require the body itself to have the tiny motor inside to drive the focussing gears in the lens. This is where the problem comes in.
For Nikon to make the D40/D40x/D60 so small, they did away with this motor in the body. This makes the camera lovely and small, but it also means that you lose autofocus with lenses that you put on that don't have an autofocus motor. This means that should you wish to use something like the 50mm (which you should, they're great fun, and perfect for portraits and playing with depth of field), you're going to have to focus manually. This is made a bit harder by the fact that the viewfinder in the camera is a bit small. So, you'll either need to buy a Sigma HSM lens (HSM is Sigma's autofocus motor in the lens), or put up with having to focus yourself.

So in summary, we've got three bodies at the lower end of Nikon's range that if you're planning on using a fixed focal length lens on that's shorter than 105mm that is a Nikon lens, is going to need manual focussing (that is, until Nikon decide to bring out some new lenses, hopefully soon!).

For that reason, I'd recommend looking at a D80. They're getting cheaper and cheaper at the moment, but if they're still a bit too expensive, check out a second hand D50, which will do the autofocussing for you. Of course, if you don't plan on getting anything but a zoom, go for any out of the D40/D40x/D60 - just remember, megapixels aren't everything!

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Oh that is so lame, every hot girl who can aim a camera thinks she’s a photographer
- Stewie Griffin