Intel socket LGA2011 CPUs have an embedded memory controller, meaning that it is the processor, not the chipset, which defines what memory technologies you can have and the maximum amount of memory that is possible. The motherboard, however, may have a limitation as to how much memory can be installed.
The integrated memory controller from socket LGA2011 processors supports DDR3 memories up to 1,600 MHz officially, but they actually support memories up to 2,133 MHz. According to Intel, the DX79SI supports memories up to 2,400 MHz.
One of the most important features of the socket LGA2011 processors is the support for the new quad-channel memory architecture, which allows the memory to be accessed in 256-bit mode for higher performance. Since each memory module is a 64-bit entity, four memory modules are needed to enable this architecture. If only two or three memory modules are installed, the memory will be accessed under dual- or triple-channel architecture, respectively. If you want to have more than four memory modules installed, you will have to install eight memory modules.
The Intel DX79SI has eight memory sockets (four at each side of the CPU socket) and, since DDR3 memory modules can be found in capacities up to 8 GB, you can have up to 64 GB with this motherboard if you use eight 8 GB modules.
In order to enable the quad-channel mode, install four identical memory modules in the sockets with the same color. If you want to install more modules, install another group of four memory modules in the sockets that use the other color.