Hi,
1) Not being able to run 2400 could also be due to CPU memory controller not being stable. Might need to try tuning manaually. I should add that if the memory controller needs the memory timings to be relaxed or increased voltages, it usually makes more sense just to stick with DDR3-2133 as it can be "faster" performance wise to do so.
2) Even if you use more than 2 DIMMs, it will still be dual channel. The additional ranks simply give the memory controller more banks to interleave. This can can help give a small performance boost in some scenarios - but this will not be visiable in any real world software in my experience. There is usually give and take between latency requirements of running additional DIMMs versus one DIMM per channel so it kinda evens out in most cases apart from some synthetic benchmarks like Super Pi 32M.
In light of the above, I'm not sure you have anything to gain from this other than spending some time and money on new parts (comes down to one's mindset if that is justifiable).
-Raja
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