well another 2x2gb will do but your cpu-mobo can benefit from higher speed ram. even if you wont manage to run them at 2133mhz, at 1866mhz will still be a lot better. just wanted to mention that.
same stuff. going faster won't make a lot of difference and if you mix what you have with faster stuff.............. besides possible conflicts it will still only run at 1333.
well another 2x2gb will do but your cpu-mobo can benefit from higher speed ram. even if you wont manage to run them at 2133mhz, at 1866mhz will still be a lot better. just wanted to mention that.