Tp start with here is the Toshiba Satellite C850 Users Guide: [http://forums.toshiba.com/tshb/attachments/tshb/brd_win7/10652/1/UserGuide.pdf|User Guide].
Here's the spec sheet: [http://www.toshiba.ca/productdetailpage.aspx?id=4452|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: [http://en.wikipedia.org/wiki/DDR3_SDRAM|DDR3 SDRAM]