What is more powerful between 2 sapphire radeon r9 270 2gb in sli or a gigabyte geforce gtx 770 4gb? If it's the sli I will need a psu of what wattage? And this is my configuration: Asus z97-a motherboard / intel core i7 4790k cpu / cooler master haf 922 case / corsair vengeance 16gb ram / kingston hyper x 3k ssd / seagate barracuda 1tb hdd / antec psu (how much watts? (I don't want to overlock)) / cooler master hyper 212 evo cpu cooler. And do the sli is compatible with my motherboard?