> 1. What is the best GB/GBC backup unit? They seem to all vary in price, are there any differences between them?
for for the GB Xchanger.
> 2. Will a GBA backup unit backup older GB/GBC games? If not, does the GBA have any trouble running flash cards from GB/GBC backup units?
no, and no. you can play GBC games on GBA with a GBC backup unit, but NOT GBA games, and you can play GBA games on GBA with a GBA backup unit, but NOT GBC games.