If you use an Ivy Bridge CPU like your i5-3570K (or a Sandy Bridge CPU), you do not need to OC your CPU in order to OC your memory. The OC memory settings are independent of those for the CPU on IB and SB platforms.
There is no specific limitation for this board regarding two vs four DIMMs, although the number of compatible/tested 8GB DIMMs is much smaller than 4GB DIMMs. The only caveat in the manual about the use of two or four DIMMs is this:
Note5: For system stability, use more efficient memory cooling system to support a full memory load (4 DIMMs) when overclocking.
In my experience with an ASRock Z77 EX 4 board, using four DIMMs that are moderately over clocked (1866 - 2000), heat is not an issue, although the memory I use can run at low voltages (1.35V) at those frequencies, and the DIMMs have no heat sinks at all. I use 4 - 4GB DIMMs just fine.