4gbddr3

I have a 2gb ddr3 desktop ramstick clocked at 667mhz and another 4gb ddr3 desktop ramstick clocked @1333mhz If i use both of them together then the motherboard underclocks the 4gb ramstick to 667mhz and i get a total ram of 6gb @667MHz So should i go for only 4gb @1333mhz or use both and get...