Yes, don't bother oc'ing your ram. You'll be better off just oc'ing your cpu. As your cpu speed increases so does your memory bandwidth. Increasing your Memory speed from, as example, 1600MHz to 1866MHz also increases your memory bandwidth. And lowering your memory latencies will also increase your memory bandwidth. So the big question is what produces the most memory bandwidth. In my experience increasing your CPU Speed will generate the most memory bandwidth followed by the Memory Speed and trailing the pack are those darn latencies.
And it's worth noting that because the Memory Controller is built into the CPU, as you increase the Memory Speed or lower the Latencies, the Memory Controller works harder and runs hotter thereby increasing the overall temperature of the whole CPU. The hotter the CPU gets the lower the overclock, the lower the overclock the lower the Memory Bandwidth.
Generally speaking if your CPU is at 4GHz and you can still run your Memory oc'd then that just means you could run your CPU faster. Depending upon your CPU and heatsink most i5 2500K's and i7 2600K's can reach 4.8GHz, after that a lot depends on your motherboard. Some can hit 5GHz+ and some don't. If your up to 4.8GHz and it just won't run any faster no matter what, then you should try raising your memory overclock as your mobo is probably your limiting factor CPU-wise.
As for your system specifically, raising your DRAM Voltage above 1.58v is not recommended because it exceeds the specification limits of the Memory Controller Voltage to DRAM Voltage Ratio. The Default Memory Controller Voltage should be 1.05v and the Default DRAM Voltage is 1.5v. These two voltage are suppose to stay within .5v of each other. If you raise your DRAM Voltage to 1.6v then you should raise your Memory Controller Voltage to 1.1v to stay at the maximum limit, 1.15v would be safer but 1.2v is the Maximum Rated Limit. Your Memory Controller Voltage will probably be called the VCCIO Voltage in your bios. I don't recommend playing with these voltages unless it's absolutely necessary.