96 CUDA Cores, 16 TMUs, 4 ROPs, Core Clock: 753MHz, Shader Clocks: 1506MHz
128-bit Memory Bus, GDDR5, Effective Memory Clocks: 3138MHz
Desktop Counterpart: GeForce GT 440 GDDR5 (GF108)
And this is where NVIDIA's mobile lineup completely loses its mind. The GeForce GT 555M is actually two completely different chips and configurations; the "A" and "B" are our designation. Our "A" configuration is essentially just a souped-up version of the GT 525M/540M/550M, with a higher core clock and the benefit of GDDR5. While NVIDIA lists both versions on their site (though lacking an explanation as to why this split was made), a glance at NewEgg suggests this "A" version is the more common of the two (powering MSI and Lenovo laptops while the "B" version resides almost exclusively in an Alienware.) You can recognize the "A" version by the use of GDDR5, but since it and the "B" version are so bizarrely matched we can't really tell definitively which one would be the faster of the two. (No review available.)

144 CUDA Cores, 24 TMUs, 24 ROPs, Core Clocks: 590MHz, Shader Clocks: 1180MHz
192-bit Memory Bus, DDR3, Effective Memory Clocks: 1.8GHz
Desktop Counterpart: None (GF106)
The other configuration of the GT 555M is a substantially beefier chip with six times the ROPs, but it operates at lower clocks and lower memory bandwidth due to the use of DDR3 instead of GDDR5. It's essentially a die-harvested version of GF106, and is identifiable by both the use of DDR3 and memory configurations of either 1.5GB of 3GB. It remains inexplicable why NVIDIA decided to use two completely different chips for the GT 555M, but hopefully this makes it a little easier to tell which is which. Raw calculations of pixel and texture fillrate suggest this "B" configuration to be the faster of the two, and as such it's probably the one to look for. Thus far we've only seen it in the Alienware M14x. (No review available.)