In going from the ipad 1 to the ipad 2, you get a dual core CPU instead of a slower single core, as well as a much faster videocard
the screen for the most part remains the same
You also get 512MB RAM instead of 256MB from the ipad 1.
With ipad 3 the memory is doubled again to 1024MB
the CPU remains the same, but the GPU is twice as fast to handle the higher res display
it also has a better camera.
PS the ipad 1 is not worth it (not enough RAM which does not make it very good for modern apps and multitasking)