How much RAM memory to install

I need to intall more RAM memory in my Alienware Desktop but I don't know the limits for this.

The motherboard is Alienware Model 07JNH0

Chipset Intel Sandy Bridge-E

Southbridge Intel X779

LPCIO Fintek F71889A

BIOS Alienware A11

Windows version is Win 7 Ultimate (x64)

How much RAM and its best clock speed can I intall in this PC?. Does anybody can answer this question as I am not an expert.