6 Cell vs. 7 Cell
 

My son has a Traxxas Bandit (non-brushless) and currently using Venom 3000mah NIMH batteries in it. I just found Duratrax 7-Cell Batteries that are 2000mah NiMH. The buggy would be faster with the 7-Cell 2000mah? but what about run time?

run time would be 2/3. 2000mah vs. 3000mah. voltage doesn't really matter unless it's such an increase you are on the throttle less. if its an electronic speed control you could even go to 8 cell packs, i'm unsure about the old mechanical speed controls, depending on the speed control(esc) you can even go to 10 cells. Lipo's would be your best bet if you plan to stay in the hobby.