Your motherboard does not support 6 or 8 core cpus.
The closest you can come is a I7 which has hyperthreading and gives 8 threads via 4 full cores plus 4 hyperthreads.
That would be a i7-3770 or i7-3770K
Here is a cpu support list.
http://www.asus.com/us/supportonly/P8B75-M%20LE%20PLUS/HelpDesk_cpu/