Guys,
The battery you use is dependent upon what electronics you mount on the nose of the helicopter. If you are the type of hobbyist that has a lot of gizmos and gadgets(switch-glo, regulator, battery voltage monitor, etc) then you will likely need a 2500-2700mah pack. If you are more simplistic like me and only run the bare minimum then you will need a 3800mah - 4000mah RX pack for perfect CG.
Hope that helps.
Thanks,
Matt