in a previous post someone posted " 1bb per mah" so hence a 1600mah battery should put 1600bb down range, though this is second hand knowledge i'd hafta agree.
my understanding is there are 2 major things to think about.
the larger the mah the longer the battery will last while shooting
the larger the spring in your gun the quicker yuor battery will get drained.
for instance a 9.6v 1600mah nimh may get 1800bbs on a 110 spring and perhaps only 1300bbs on a 140 spring.
so if possible get as many mah in your gun as you can get