Any surplus power resistor in the range of 15 to 20 Ohms will work fine for a 4.8 Volt pack of nomenal capacity. If you are into more servos and higher current applications you may want a lower value resistor. Generally I like testing at a current of around 40% of battery capacity. Graphing the discharge curve for new batteries and comparing to the original graph as the pack ages will keep you out of trouble. Pay attention at the end, if it drops suddenly when the voltage is at 4.4 or higher you have a weak lower capacity cell. A good pack will get below 4.3 before hitting the discharge knee.

Last year I started charging my systems with a method proposed by Red Scholefield. I plug everything into a timer set for one hour per day to keep all the batteries fully charged. If I come back from flying, I charge normally overnight, then go to the one hour per day at full wart current. Works really great, costs less than $10 for the timer.

I highly recommend you spend some time reading this web site:

http://www.rcbatteryclinic.com/

I'm pretty sure he has forgot more about NiCad's than the rest of us will ever know. And he didn't forget very much.