The RAM multiplier options of i5-750 are 6x, 8x and 10x and so you want to get a RAM that can run at spec with your current BCLK and available multiplier.
Hence, your choices are:
(with 4GHz=200x20 CPU frequency)
In addition, lower frequency RAM are usually with lower latency too which is better than RAMs that have higher frequency and higher latency.
Thus, 1600MHz ram is your best choice.
I would get a 1600MHz & CL8 RAM, because it has the best cost-performance ratio.
However, don't be hesitate to go for CL6 and CL7 if they are only a few dollars more than CL8 one.