Can I run x2 4GB sticks and x1 8GB stick? Will running in single channel significantly hurt performance?

Thun3r

Distinguished
Dec 25, 2012
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My system:
CPU: FX-8350 Black Edition
MB: Asus Sabertooth 990FX R2.0
GPU: 280x/7970 in CF
Ram: 2x 4GB Corsair
OS: 64bit Windows 10 Pro

I use my PC for homework, Chrome, and games. I am upgrading my ram for Fallout 4 and Arkham Knight.

My current memory type is: G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-8GBXL
(Pretty sure this is running in 1333. I have no idea why.)

If I bought: G.SKILL Ripjaws X Series 8GB 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Desktop Memory Model F3-10666CL9S-8GBXL to go along with my current memory would I have any issues?

Would I notice any decreases in performance after going from dual channel to single channel?