8gb 1600mhz + 2x4gb 1333mhz, have 4 slots, best configuration?

Tradesman1

Legenda in Aeternum
and how much DRAM do you want/need - if it supports it and isn't a 2011 socket, then best to get full amount in a 2x config (2x4GB for 8 or 2x8GB for 16. On a dual channel mobo only use all 4 slots if maxing out DRAM (generally 32GB with most DDR3 based mobos)