AMD CPUs have an embedded memory controller, meaning that it is the processor, and not the chipset, that defines the memory technologies and the maximum amount of memory you can have. The motherboard, however, may have a limitation as to how much memory can be installed.
At the moment, the integrated memory controller of socket AM3 processors supports only DDR3 memories up to 1,333 MHz under dual-channel architecture, however ASRock says 890GX Extreme3 supports DDR3 memories up to 1,800 MHz through overclocking. 890GX Extreme3 has four DDR3 sockets and since, at the moment, each DDR3 memory module can have up to 4 GB, you can have up to 16 GB with this motherboard.
The first and the second sockets are white, while the third and the fourth are blue. In order to achieve the maximum performance, you should install two or four memory modules to enable the dual-channel architecture. When only two modules are used, install them in sockets with the same color in order to enable this feature.