Officially X58 only supports 1066MHz RAM. In reality though it can top 2000MHz if you have good RAM and give it enough juice.
Your 1600MHz RAM will be fine - just make sure your multipliers are set correctly in the BIOS (but if you're just dropping in a new chip to an existing board it should already be set properly).
Also, it might be worth checking you have the latest BIOS version for your board so you know it can support a 6-core chip - if not, do a BIOS update before you change the chip. However if you're running a 970 already you'll be fine as the 970 is a 6-core (the 950 is a quad so you need to be sure which one you have).