i just wanted to tell everyone, that i got the display working. there is still much for me to do. cleaning up the code and doing the setup again with a proper voltage regulator as pointed out by massimo. the diode-method works, but i'm not satisfied with the contrast and dirtiness of this solution.
the main problem still is that the code to use the display is quite big. i managed to interface a joystick with the same board, to control a cursor. every further processing has to be done externally on another board or a computer. so goodbye DIY arduino gameboy 
if anyone is interested in the code or setup now, just PM me.
kuk