I am building a Sanguino now, need more memory and more digital/analog pins.
Same here. I switched from a 20x4 character display to this, and I've already run out of pins. So now I'm waiting for the 644 I've ordered. I'm cheap so I'm just going to build it on a breadboard, using a Freeduino for power supply and programming.
When i have completed the Sanguino i will add buttons to control a menu and a Accelerometer to measure the g-force.
Same here 
I'll use the accelerometer to measure lean angles and G-forces, and also to activate the brake light.
What kind of protocol are used for a motorcycle?
A few bikes (BMW and Ducati that I know of) are using CAN-bus, but this is just a plain Suzuki and I'll use another AVR168 to collect data and transmit it to the display-unit using serial or I2C.