The RDS8000 has a low voltage cutoff of something like 2.5 volts. You will see problems with your servos long before your receiver shuts down. What this means is that a 5 cell, 6 volt pack is definitely not needed, just an option if you want faster servos with more torque.

I have the same radio. I use a 1600mah pack for the radio itself and it lasts a good two days of flying without needing a recharge. I got the battery from RadicalRC.com.

You can get 2000mah battery packs and that should last you quite a while, again probably two days of flying without a recharge, for your receiver.

People do run two "redundant" switches and batteries, but usually on larger, more expensive aircraft. If you want, you can do something like two switches with two 1000mah battery packs. This would give you added protection and would be approximately the same flight time as using one 2000mah pack.