I'm not too sure about lightroom. Basically though, I would go with the 750 for the quad core. If the program uses multiple threads, the 660 would be a downgrade. From the description of the program though, I doubt you would notice much of a difference with either. But I've never used it so...
Please also note that you will not be able to upgrade just the processor. The i5 would require a new 1156 motherboard and DDR3 ram. Just to make sure you know that.