Power, Temperature and Noise
Dual GPUs add approximately 150W over and above a single card's system-wide output. 400W at the wall certainly isn't overkill for a machine with a 4.4GHz Core i7-4790K and two overclocked GeForce GTX 970s providing the graphics oomph.
EVGA's ACX 2.0+ cooler is sufficiently capable such that it keeps two close-proximity cards from reducing maximum in-game frequency too much - a single card peaks at 1,392MHz core while two manage 1,379MHz.
While the duo remain silent when idling, positioning them fairly close together on an Asus Z97-A motherboard does increase fan noise, which runs up from 1,200rpm to 2,000rpm on both cards. Noticeable? Yes. Bothersome? Not really.
