All the cameras suggested have merit. However it might help if you can say a bit more about why you find the D40 limiting and what you find it does well.
Ah, yes, that's a very good point.
well, I suppose I find it most limiting when I crop shots down in PPing and the 6MP sensor lets itself down a tad. The image quality could be better. it's not as bad with my 300mm, but still pixelates the same.
I also find the that the FPS rate can be a bit tedious at times as I tend to shoot a lot of motorsport and always in RAW, something quicker would definitely be a benefit.
for this reason, also, more AF points maybe useful for tracking moving subjects.
I also find it a little bit on the small side, I wouldn't mind something slightly bigger.
wireless flash command would be nice,
AEB would be really nice as I like to shoot HDR, but I know the D3100 doesn't have this. I can still continue working from a single RAW as I do now, but AEB would be handy. it's something i'd use.
Movie mode is also a big selling point as our First child in on it's way (due in Jan). I think it would be nice to capture some "first moments", if you know what I mean.
The ISO shows noise at 800, i've read that others have much higher ISOs and show less noise, maybe again due to the sensor?
Likewise, most other SLRs will now drop to ISO100, the D40 will only go as low as ISO200 (not that i've found this limiting in any way, as yet).
I liked discovering and learning new things about the camera, but I think i've hit a bit of a brick wall with the D40, and can use all of it's features (not that there are that many).
LV would be useful for shots very low down or above head height. (i've found myself in situations where it would have been great).
DOF preview would be nice, but not essential.
Screen on the D40 is small, would be nice to have something bigger.