Tp start with here is the Toshiba Satellite C850 Users Guide: User Guide.

Here's the spec sheet: Toshiba Satellite C850

Your system uses: PC3-10600/12800 DDR3-1333 SODIMM 204pin memory. As to the memories speed You can use a faster rated module (1600MHz) but you shouldn't use a slower than what is spec'ed. In your case the speed needed is 1333 MHz.

Here's a little more detail on DDR3 memory: DDR3 SDRAM