The 670 is based on the Kepler architecture. The 750 Ti is based on Maxwell. The Maxwell architecture is much better for hashcat. Even just one 750 Ti would be better than a 670. And 2 750s would be even better. 
My 750 Ti benchmarks MD5 at 3345.1 MH/s. I do not have access to 670 benchmark data. [Edited to add:] So it would be a good idea to get some real benchmarks first, especially considering the difference in cores, etc.
My 750 Ti benchmarks MD5 at 3345.1 MH/s. I do not have access to 670 benchmark data. [Edited to add:] So it would be a good idea to get some real benchmarks first, especially considering the difference in cores, etc.
~