can anyone tell me please the differences between
apart from the megapixels
F31fd is the best camera of the listed group straight out. The F4x series would be next in terms of image quality and high ISO capabilities. The F50 is a very nice camera on paper but the quality at and above ISO400 is extremely poor compared to the class leading F31. If you can find a new or refurb F31 online, pick it up right away and you won't be disappointed. You'll probably be able to sell it at a profit a year or two from now because of Fuji's moronic move to joining the megapixel race.
Another note on the F50 is the vertical banding issue at higher ISOs and in darker situations. You can see it on some ISO 400 images but if you do anything in 800 or above it's very noticeable and highly disturbing. You can push the F31 to ISO 800 without much of a problem, and easily hit ISO 1600 in a pinch for every day images of friends/family unless you're a pixel peeper.
The F40 is a nice camera, kind of in the middle between the F31 and F50. It doesn't have Shutter/Aperture priority modes though like those cameras. Has more pixels compared to F31 so noise is about 1 stop worse than the F31. Still pretty good to ISO800 though. If you can't find an F31 go for the F4x model as long as A/S priority aren't super critical. Hope this helps.