here is a circuit showing a uln2003 driving a motor, it connects the same as the uln2803.
Ignore everything in the diagram but the uln2003, and just connect your microcontroller outputs to the left side of it where the 4 wires are now.
The 500ma rated current for the darlington array is for a single darlington set(pin), so if you had a couple of darlingtons you can probably connect them in such a way as to share the load between more than 1 pin. Before you do this, definately make sure that your stepper is connected properly otherwise it will just increase current draw