Example - solve the speed and will it be problem free?
667 5-6-5-20 @ 1.5v
533 4-5-4-20 @ 1.8v
Memory is a 'finicky bitch' and needs to be handled 'right' -- my version is 'Do to Right...' ...can mixing and matching work -> yes, is it smart no.
BTW - Worst, in this example, Dell is a PITA and locks-down the BIOS to properly set all values.