animation design for a platform with little ram

[sefiroths]

I'm developing a game to iphone using openglES.
However I guess the problem is general:
I have a 2D character who does things.
the sprite of the character is almost as big as the whole screen suppose 512x1024 pixels
I have to make an animation from (eg) 20 frames do not send out of memory in the phone.
I had thought of separating body and limbs inmodo that as the body moves a little (I can make 4 frames), arms (20 frames), legs (10 frames).
if I make a spritesheet, maybe I'm within the limits of ram ...
But after re-compose the image starting from spritesheet becomes a hell ...
is there any tool that makes the purpose?
something like: I have several frames of animation, cut-out pieces, meanwhile I made a header that tells me that the position they were ...
someone who has other ideas on how to tackle the problem?

thanks