After doing all that you, sir, told me to do I'm still not able to get the option on the software.
First, I cleared the CMOS and updated the BIOS. Then I installed onboard graphics driver v.8.982 (The one that drivers CD provides) and upgraded it to v.12.102 (The latest one from here (http://www.gigabyte.com/products/product-page.aspx?pid=4384#dl)). After that, I installed the GPU and changed the BIOS settings to the ones you said I have to. Finally, I installed the latest driver of my GPU (http://sites.amd.com/us/game/downloads/Pages/radeon_win7-64.aspx) and these are the options I got when I go on the Performance settings of the AMD Vision Control Center:
(http://i.imgur.com/dCMAGtx.png)
What I'm doing bad? Because I have tried so many times to make the Dual Graphics work and it still doesn't.