Hi Richard,
your motherboard should take 4GB RAM Modules as the specifications state:
4 x 1.5V DDR3 DIMM sockets supporting up to 16 GB of system memory
If you are looking for an upgrade then I would suggest replacing all of the modules and keeping your current set as spares as it is never a good idea to mix and match.
Corsair Vengeance modules are good value at the moment, are the right voltage at 1.5V and seem to be the best in various tests I have done on different AMD Gigabyte motherboards. I am currently running a 16Gig set on my GA-990FXA-UD5 although they are not on the QVL they work perfectly.
