I would go with an extra set of the same RAM in 2 x 4 gb's to add to your existing set (it is cheaper that way). If you want to totally replace your set, I would go with 2 x 8 gb sets, so you have the full upgrade path to 32 gb's, if you so desire later.
According to GB's website the RAM is:
4 x DDR3 DIMM sockets supporting up to 32 GB of system memory
* Due to a Windows 32-bit operating system limitation, when more than 4 GB of physical memory is installed, the actual memory size displayed will be less than the size of the physical memory installed.
Dual channel memory architecture
Support for DDR3 1600/1333 MHz memory modules
Support for non-ECC memory modules
Support for Extreme Memory Profile (XMP) memory modules
(Please refer "Memory Support List" for more information.)
So the most you can have is 4 x 8 gb's of RAM for a total of 32 gb's.