You absolutely shouldn't fly 4.8v receiver batteries with Spektrum radios. 2000Mah 6v NiMH is a good minimum battery size for sport planes with Spektrum/JR radio setups. A large capacity 6v receiver pack still can't guarantee against occassional lockouts or brownouts, but it will greatly reduce them.

On a high performance plane with 6 or more digital servos, you should consider a 2S or 3S LiPo battery with a 6v voltage regulator instead of NiMH. High torque analog and digital servos can draw more current than many NiCd/NiMH batteries can deliver reliably. LiPo and LiFe (A123) batteries are becoming very popular as receiver batteries for high performance aircraft.