It also depends on the duration of your flights, if they are mostly simple circuits, or extreme aerobatics (more servo movement)and number of servos in the model.
The 2000mah Rx Nimh pack is a good suggestion. You want to be able to charge it at arount 150 - 200ma's.
Another advantage of Nimh over NiCd is the metal hydride cells do not develop a memory like nicads do.