The Gigabyte GTX 650 OC 2gb card is the best among those two.And remember one thing the nvidia cards dont use much power and produce less heat compared to the amd cards.And the GTX 650 is 2gb.So you can get more benefit with it.I'm using HD 7770 1gb GDDR5 since 2months and i'm not satisfied with it cause 1gb is too low for the present games.So you need 2gb.So the best is to choose the GTX 650.And 400 watts would be sufficient for those two cards if it is a nice one.You should have the PSU from branded manufacturers like Corsair,Thermaltake,Seasonic,Cooler Master,Gigabyte and some other.And if you even have a low branded power supply,your system would be fail to run it and you may need to change your psu again.So better get a corsair 450...