Your computer can't use RAM at different speeds. It needs to get its data at a somewhat even rate: if you get one byte in 30ns and the other takes you 42ns, then your CPU can only process the data after its got the whole thing, that is, after 42ns. So... You can USE 4200mhz RAM along with 2400mhz RAM, but you have to run it at the lowest speed and highest latency settings supported by any of the sticks.