If I want to employ all four RAM slots in the GA 790FXTA-UD5, am I correct in my understanding that I am limited to 1333 Mhz, while if I elect to use only two slots, I can attain 1866 Mhz with suitable modules? Supposing that I purchase two 2 GB 1866 Mhz modules, and wanted to add a further two in the future. Would it then be possible to add two 1333 Mhz modules or would they also need to be 1866 Mhz?