Actually, weight is a secondary concern. What good are the lightest batteries in the world if they can't deliver enough current to run the motor?

When powering a plane, the first thing you normally do is look at the batteries. In this case, the plane was designed around a specific power system, so you should use that power system.

8 Volts? Probably 8 CELLS. I bet that's what the instructions really say. Batteries go by cells, because the Volts can vary from .08V up to 1.4V from empty to full on a NiMH/NiCd.

The Speed 400 power system is probably one of the most mature successful power systems out there. If you've got a Speed 400, run an 8-cell, 2/3A NiMH battery pack. 2/3A refers to the cell size, of course. Same diameter as an A (little bigger than AA) but only 2/3 the length. In NiMH this size cell has 950-1100mAh of capacity depending on the manufacturer. Some options include the KAN1050, GP1100. For Speed 400, a pack this size is the ONLY way to fly, pardon the pun.