Amazon.com: AMD Radeon R9 280X 3GB HDMI PCI Express Video Graphics Card for Apple Mac Pro, outperforms ATI Sapphire Radeon HD 2600, 4870, 5770, 7950, 9000, 9800 Gigabyte : Electronics
AMD balances Radeon deck of graphics cards
GPU Nvidia N13E-GT-W-A2 Graphic Video IC Chip for iMac 27" A1419 (Late 2012)
NVIDIA GeForce GTX 660 Ti Graphics Card: Affordable Graphics Powerhouse | Windows Experience Blog