As stated in the main Emulation Legality page, this is a complex issue (and differs across jurisdictions throughout the world). Anyone wishing to make backups of their own games is urged to conduct their own research before proceeding.
Stances towards the use of emulators and images/roms/iso’s differs from country to country, from hardware manufacturer to manufacturer, and from software developer to developer.
Nintendo, for example, provides detailed information regarding that company’s standpoint with regards to the use of emulators and game images:
The Pi 2 has been overclocked to extract the maximum performance possible, as many video game system emulators push the hardware to the limits. Please see the series of posts on overclocking and stability testing, beginning with part one, for further details.
Micro SD memory card
SanDisk SDSDQUN-032G-FFP-A Ultra microSDHC UHS-I Class 10 Memory Card SanDisk 32GB micro SD (Amazon)
I’ve had mixed success with compatibility of cards in the Pi 2 – most have worked; one 16Gb card was unstable under Noobs and Raspbian, but fine with the RetroPie image