Radeon R7 250 is better because the clockspeed is 1000mhz instead of 900mhz by the Geforce GT640. And the memory speed is 1150mhz instead of 891mhz and the memory performance of the Radeon R7 250 is 26% better in performance but the recomended PSU is 400 watt for the radeon and 350Watt for the Geforce.