CONFUSES & SOLUTIONS

I know it is gonna be a model for game, not for animation so I will get away with glossy, great-looking renders with massive polygons.
But I wonder how many polygons should be put up on screen for a game? And to apply it for what platforms? (i.e: Ipad, Window, Xbox...)
And put on up those questions with pen. Also the possibility of making from this to

...this with normal map

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
No I can't make it since the two model are too different, so the normal map won't reach. Instead Ben suggests to sculpt something in low-poly model then export back into Maya-------> do the UV map -----> export to the MudBox and now work on the higher-poly version.

Well, I should try things out before any further questions with Ben. Thank you Ben.