I will be crossfiring the cards not sure what it really is but i know the simple fact that its great and makes graphics run smooth as heck, not sure if they will go together and what not with the motherboard or even together themselves.
Before ordering the asus board, download the board manual and read it. If you don't have the time and patience to simply read the manual, then I wouldn't build your own. I remember my first builds and how long it took to figure out simple problems. The 8120 cpu is still fairly new, but some boards may require a bios flash to get it running. Go to the gigabyte webpage for the board and see if any bios files cover newer cpu codes. They're listed in their own section. If not, then it may run fine right out of the box. And leave the board and power supply out to test the components on a cardboard or wood surface, typically known as "breadboarding". It's far easier to see if everything works before mounting it in the case. Good luck.
o1die i think means to say go to the Asus webpage for the board and see if any bios files cover newer cpu codes. They're listed in their own section. for GPU between the R6870 and R6950 go with the R6950 Also the PSU you picked is not very good 750w is recommended if you are going to OC.