Udo Klein:
Ok - so you -are- basically doing what I am talking about - that's pretty cool.
My thoughts were to make a general purpose engine with the Arduino (well, an ATMega328 w/ bootloader), to handle sprites, tile mapping, scrolling, etc - and this "render engine hardware" could be sent commands and data via SPI or I2C by another Arduino or any uC, really. Perhaps such a device could be implemented as a "backpack" for the LCD.
That would make a pretty killer product; current serial backpacks either don't have the speed, or have a limited library for graphics (enough for simple GUIs and the like, but not for games and "high speed" graphics creation).