I have a Sabertooth 990FX Gen 2.0 motherboard with FX 8350 using Corsair 1600 Mhz Memory without any issues.
I would download the Asus 990FX motherboard RAM Memory QVL list and see what memory they have that is compatible with your motherboard.
In my opinion, a slower RAM memory shouldn't be an issue with a CPU. Your computer may just be a little slower with that low of a RAM Speed installed. If your RAM can support OC through BIOS, Then you can OC the RAM automatically using the OC Profiles for RAM. Make sure your CPU is compatible with what your Motherboard will support by making sure you have the correct BIOS version installed to be able for the motherboard to recognize the new CPU.
My QVL list shows RAM Memory of 1333 Ram Speed for my Sabertooth 990FX Gen 2.0 motherboard.
