Good choice. I along with Tripp can vouch for the Gigabyte GA-990FXA-UD5 motherboard. I have it myself for nearly a year now and it's been rock solid stable since day 1.
Just to clarify, you can pair AMD CPUs with Nvidia GPUs no problem. Nvidia still wants to be able to sell you their GPUs if you're one of the 25% of people running AMD CPUs (according to Steam's HW survey).
Just like you can pair AMD GPUs with Intel no problem, AMD wants to be able to sell their GPUs to everyone aswell regardless of whether you're on AMD or Intel for the CPU.