Someone at my club suggested I switch from nickel metal hydride to lithium ion (not lithium poly) batteries for longer life. I have a Futaba 7C 2.4GHz radio and R617FS 7 channel receiver. Two questions:

Can someone recommend a good brand and voltage/amperage rating to go with for long life and fewer required charges?

Can I use a 7.4 or 6 volt instead of 4.8 volt, in addition to a high amperage like 2400mah without making any other changes?