Question about getting extra RAM

I currently having a single channel 8GB DDR3 RAM 798MHz and want to get another one. Does buying another 8GB ddr3 with same speed enough even if they are not same brand/model? If I can't find one with the same speed is is it ok to buy one with different speed? I only one slot available for extra RAM. PC specs on my signature
 
Usually if you get a 2nd module you TECHNICALLY shouldn't need to worry about brand (if only it was that simple). The timings and speed are of key importance, although FASTER RAM is usually compatible with slower RAM if you run everything at the slower RAM's speed/timings (the BIOS will normally configure that for you automatically). Bit of a lottery, but you'll PROBABLY be fine if you buy a second stick of the same or better spec than your current stick.

EDIT: Oh I see from your system specs it's actually DDR3 1600.
 


My computer is pre built so kinda hard to find the exact RAM stick.