Do you have the exact steps/procedures from you mobo on how to turn the onboad GPU of the CPU off.
As I noted above, it should be automatic. But to be sure, I just checked the manual for my GA-Z77X-D3H board and as suspected, the BIOS options for "Internal Graphics" are:
Enabled
Disabled
Auto (default)
So, if you leave the BIOS settings alone and you install a graphics card, the BIOS will automatically disable the on-board and boot to the card.
I've been using Intels on all my builds since the Core 2 Duo came out in 2006 with no regrets. And I mostly use Gigabyte boards too (though sometimes ASUS), also with no regrets.
As a whole line of products, Intel CPUs generally provide better performance while consuming less power and generating less heat - with a few exceptions, of course. That said, AMD makes excellent CPUs too so either can be used to form the foundation for a great, reliable computer.
Much is made about AMDs costing less - with folks saying you get more bang for you money. While that may be true, once you factor in the price of the motherboard, case, RAM, power supply, drives, keyboard/mouse, speakers, monitor(s), a "good" UPS with AVR, and OS, then stretch that over the life of the computer, IMO any difference in cost is a wash.