You can use any 670
However if you get a regular 670 which is not factory OCed, than your Evga 670 FTW will work at the other GPU's speed.
Just as long as it is a 670 and same VRAM amount than no problem at all.
Yes. You can OC both of them to the same speeds unless one of the GPUs are not very OCable.
Not every GPU can OC to the same level.