What exactly is the diffrence between these video cards

NVIDIA GeForce 660m

and

NVIDIA GeForce 675m

 

NVIDIA GeForce GTX 675M - Notebookcheck.net Tech

 

big difference.
660m is based on the new Kepler architecture built on the 28nm node, so basically, it has twice the number of shader cores as fermi but each core runs at half speed.
the 660m has 384 CUDA cores running single pumped at 950-1085mhz with a GDDR5 memory system providing 64GB/s bandwidth. The core also has 32 Texture units and 16 ROPs.

The 675m is based on the older Fermi architecture, it has 384 CUDA cores at 620mhz but they run double pumped so the effective speed is 1240mhz. Memory subsystem is GDDR5 providing 96GB/s bandwidth. The core has 64 Texture units and 32 ROPs.

In terms of performance, the 675m shouldn't be too far ahead of the 660m in Shader bound games considering the roughly similar number and clockspeed (i.e. most console ports). However, the 675m has significantly greater pixel filtering capabilities coupled with greater memory bandwidth therefore it takes a smaller performance penalty with increases in resolution and/or usage of MSAA (e.g. Crysis). The 675 also has greater texture filtering capabilities therefore it takes less performance penalty with Anisotropic filtering.

In terms of power consumption, the 660m has a real edge due to the 28nm process, the TDP is fully half that of the 675m therefore it generates less heat and consumes less power at full load and idle. The 675m also has a greater PCB footprint due to its larger memory bus (256bit vs 128bit) and bigger die therefore it is very difficult to deploy in smaller machines.

The 660m should have greater OC headroom due to the 28nm process consuming less power and outputting less heat (lower TDP). However, it will likely be bottlenecked by its ROPs and Texture output whereas the 675m will likely be restrained by heat and power consumption.

 

Biggest difference is the 660m is a 128-bit card and the 675m is a 256-bit card. That translates into higher resolutions will be problematic for the 660m. For 1366x768 or 1600x900 even the 650m/660m will perform admirably, but performance will degrade quickly going up to 1080p (or higher).

In any case if you're opting for either, perhaps see if you can get the 670MX or the 675MX since they are refreshed with the newer Kepler architecture which means they will run cooler, consume less power, and likely have more overclock potential.

 

Yup like WingNut said, if you can upgrade to the 675MX, that will be your best bet (or 680m if your budget allows for it). If you are stuck to ONLY the two in your OP, then get the 675m.