GO Plus System Requirements have also been given
iOS
Operating System: iOS 8 - 9
Models: iPhone 5/5c/5s/SE/6/6s/6 Plus/6s Plus
Android
Operation System: Android 4.4 - 6.0
Requirements: 2GB of RAM, Bluetooth Smart (Bluetooth Ver. 4.0 or higher)
Niantic have also put out a list of devices they have tested the device works with. Other devices should work if they match the requirements, but there is no guarantee
iOS Devices: iPhone 5, iPhone 5s, iPhone 5c, iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone SE
Android Devices: AQUOS Xx2 mini, AQUOS Xx3 506SH, ARROWS Fit F-01H, Galaxy A8 SCV32, GALAXY Active neo SC-01H, GALAXY Note Edge, Nexus 5, Nexus 6(XT1100), Nexus 6P(Softbank), Qua phone KYV37, TORQUE G02 KYV35, Xperia Z2 SO-03F, Xperia Z3 SO-01G, Xperia Z5 Compact SO-02H
09:58: We also have details of how the game looks on the smartphone when the device is connected. When it is, an image of the GO Plus hovers on screen. When you're in range on a Pokémon or a PokéStop, when the Pokémon GO activates and detects it, it will point it out on the screen showing what the GO Plus is pointing at. This is useful if you are in range of multiple things at once so you're able to see what the GO Plus is activating if you have your phone open