EVGA has higher clocks but gigabyte has better cooling.but clocks will not affect much.you can overclock your gigabyte card too.580 Sli is a beast setup.i would say go gigabyte.SLi produces more heat and you need a better cooling which gigabyte has
I think that would be a swell choice. Gigbytes a good GPU maker, but i don't like the Color they put on their PCB's, i had a Blue PCB Gpu one time and had nothing but system problems. Not saying it has to do with PCB but just brand wise. and all my Black PCB cards have worked Stellar
Definitely go with evga for the lifetime warranty and there support. Just be warned the gtx 580 classified seems to have some mixed reviews. I was orginallys going to purchase one, but i guess they get really hot and there alot louder because the fan is more powerful. I ended up going with the 580 SC and couldnt be happier.