Every one has a starting point, no worries.
Your motherboard supports up to two video cards and is optimized for AMD (ATI previously) multiple cards. Other than AMD video cards will work fine but are just not optimized. Honestly, I don't know the difference it could make.
Multi GPU setups basically require the same video cards installed. It is okay to be from different video card board manufacturers. Just make sure it has the same chipset and preferably the same amount of video memory because it will use the minimums shared amount of both.