If you mean -
N660-
2GD5
N660-
2GD5/OC
N660 Gaming
2GD5/OC
N660 TF
2GD5/OC
Then it is most likely a reference to the 2Gb of onboard Vram, especially given that the 4Gb GTX680 does by this name -
N680GTX Twin Frozr 4GD5
The D5 is a reference to GDDR5 memory.
ScrewySqrl :
Twin Frozr is MSI's brand name for having two fans - Twin Frozr 3 means 3 cooling fans
Twin Frozr 3 just means the third version of Twin Frozr, still two fans. 700 series Nvidia cards by MSI are on Twin Frozr 4, still two fans.