I've had infinite not-booting issues even on using two of the same RAM sticks bought without a set, and endless random BSODs with glitches in everything while mixing two different frequency RAMs.
Mixing RAMs barely worked for me. You'll be better off selling or keeping spare the RAM you have right now and buying a set of 4 RAM sticks, according to your budget. A set, because then their clock timings are pre-set and tested and they're most likely to work if everything's compatible.