With modern processors, RAM speed makes more difference than RAM timings. The DDR3-1600 will perform better on a Sandy Bridge system.
I no longer recommend 4GB of RAM on any new system. 6GB is the minimum I recommend while 8GB would provide a bit more longevity. They provide smoother performance while gaming and doing normal multitasking, and RAM is cheap.
7, 7, 7, 20 DDR3 at 1,333MHz is faster than 9, 9, 9, 24 DDR3 at 1,333MHz.
7, 7, 7, 20 DDR3 at 1,600MHz is faster than 7, 7, 7, 20 DDR3 at 1,333MHz.
To figure out which is faster, so far as a combination of frequency and timings, the general formula is:
x = CAS (first number in the timing set) ÷ Frequency
a) 7, 7, 7, 20 DDR3 at 1,333MHz. 7 ÷ 1333 = ~0.00525
b) 9, 9, 9, 24 DDR3 at 1,600MHz. 9 ÷ 1600 = ~0.00563
Therefore, a is faster than b, as a has a latency 7.2% lower than b. It takes the least time to perform an operation.
When overclocking, to take the RAM out of the equation, set loose (high) timings and low frequencies, so you are effectively running the RAM at quarter throttle. Once you have your CPU overclock steady, gradually increase RAM frequencies and/or lower RAM timings to get the fastest possible RAM speeds.