What kind of airsoft battery should i get for an Echo 1 g36c?

ok i am soon going to be getting an Echo 1 G36c, and i have heard that the stock batter that it comes with sucks. i have done some research and i understand the difference between NiCd and NiMH but what is an "intellect" battery? what difference will i notice between batteries with the same amperage but different voltage (8.4v vs 9.6v)? same thing goes for amperage. what difference will i notice between 2 batteries with the same voltage but different amperage (say 1100mAh vs 160mAh)? and why do i occasionally hear things like (do not use this battery with a stock gearbox)? lasty i would like to know which batteries will actually FIT in a g36c w/o using a battery bag or something like that.