First very few notebook's allow for OC'ing RAM greater than the CPU's default IMC 'SPD' and if you're thinking about mixing both the 2GB DDR3-1600 together with a 4GB DDR3-1333 then I never advise mixing different Frequency & CAS RAM together. Further, >4GB RAM requires a 64-bit OS.
You can see what 'SPD' your current RAM is running using CPU-z and look at the 'Memory' (tab); e.g. DRAM Frequency = 800MHz (DDR3-1600) or 667MHz (DDR3-1333). CPU-z - http://www.cpuid.com/softwares/cpu-z.html