You'll be better off buying a "kit", i.e two sticks of RAM that were tested together by the manufacturer. Mixing RAM, even from the same manufacturer with same part number, can have problems working together due to manufacturing process variations. Two modules from different manufacturers are highly unlikely to work together and if they work at all they will probably work at a slower speed and as "interleaved memory, meaning you give up the benefit of the the dual channel memory bus.
Each module has a 64bit wide data path. With different modules that do end up working together or with a 1X8 and a 1X4 used, that data path will be treated as one 64 bit wide bus.
With the matched pair of a kit working together in dual channel mode the data bus will effectively be 128 bits wide because they can read/written at the same time. So the memory will appear to be twice as fast because it only takes half as many read/write cycles to read/write the same amount of data.
That's assuming you have a dual channel motherboard, which you do. If you had a triple or quad channel motherboard the effective bus size would grow to 192 and 256 bits wide, respectively.