I going to by a digital radio control, do I need to buy a digital servo? I my radio receiver on the truck has 4 aa batteries. I am told that I can buy a rechargeable battery kit that will increase my throttle response and braking and I think turning. Not sure what to really call this battery device. Any help would be great.

Kinda confused. What radio do you have now and what one are you considering. Do you mean you have a AM radio now and plan on buying a 2.4 radio? Digital servos are always a better idea. But some analog servos have operating issues with 2.4.
Rechargable AA's honestly only save you cash in the long run. They offer no added performance. Actually alkaline AA's have a higher voltage than Nimh AA's(recargable). But Ive been using rechargable AA's for along time with no problems