The following is general advice for all cards.
Your PSU should be rated at or above the minimum wattage recommended for your GPU
The PSU should have an amperage rating at or above the required minimum for the card (12v) rail.
Its best to uninstall the existing drivers, shutdown, remove old card, install new one then restart and install drivers for the new card.
The new architecture of the 1070 yields 20-45fps (depending on res) over the 970. Not to shabby.
