12GB ddr3 1333mhz or 8gb 1866mhz ddr3 for games?

If you are using integrated graphics, then faster 1866 will be better.

If your cpu is intel, speed matters little.
If your cpu is amd, then speed matters more.

Few games are going to use more than 2-3gb by themselves so more than 8gb is not all that helpful
If you will be multitasking while gaming, then 12gb will keep tasks from interfering.