Apple has radically overhauled its iMac lineup for 2017, giving its 27 inch Retina 5K models dedicated graphics driven by AMD Radeon Pro 570, 575, and 580 GPUs; a brighter display with enhanced P3 wide color; more powerful Intel Kaby Lake CPUs; upgradable RAM sockets and modern, ultrafast USB C and Thunderbolt 3 connectivity.

Source:: Apple Insider