Something in the 60-70 oz/in range should be ok.......check these out if you are on a budget. I have some and they have been fine.

http://www.servocity.com/html/hs-475...er_pro_bb.html

If you look at the specs, you'll notice that on 6V, they are not only faster, but almost 20% stronger as well. Buy a NMH RX pack with 5 cells. It will work fine on your wall charger as well as on the NMH setting on your field charger.