8 is sufficient for gaming now, but if in the budget and depending on what you do other than gaming, I'd go 16 (again if in the budget). Haswell likes fast DRAM so I normally suggest 1866/9 as the entry level:
The 4670K is 'native' to 1600, but runs 1866 fine with no OCing or additional voltage, many of the lower end Haswells may require additional voltage or OC the MC