The higher the number at the end, the faster the memory. So GDDR5 is the fastest of those three (and it's usually a multiplier - so GDDR5 is 5x data rate whereas DDR3 is only 3x). The G is supposed to signify designed for GPUs, but often just is added for graphics cards with no real change in design. So DDR3 roughly equals GDDR3, and GDDR5 is faster. Thas said, you won't find main RAM labeled with the "G"