When I was looking at my current system I came across this as like you i had questions on memory speeds.
On platforms which have integrated memory controllers, the CPU, motherboard, and memory all have to support common speeds. If they do not, one or more components will have to be overclocked to match the others.
Bulldozer has native support for DDR3-1866. This means that on an AMD motherboard which also supports DDR3-1866 it should have no problems running memory that is rated at DDR3-1866. However, I have seen some people have problems running rated memory on Bulldozer.
Intel's Sandybridge 2000 desktop processors only have native support for DDR3-1333. This means that DDR3-1600 involves overclocking the memory controller. 98% of all chips will have no problems running DDR3-1600, and at least 90% should be able to run DDR3-2133 which is the highest standard profile. There are always a few lemons though which won't be able to run these overclocked profiles and since they're not "officially supported" there's no recourse for the consumer. This means that DDR3-2133 memory on a motherboard that supports DDR3-2133 may not actually run stably at DDR3-2133 if the CPU isn't of particularly high quality.
Now, there's a massive performance difference between DDR3-1066 and DDR3-1333, and a massive performance difference between DDR3-1066 and DDR3-2133 (a 100% increase in memory IO bus clock and memory core clock yields about 33%-50% improvement in some applications) but the gains diminish very quickly after DDR3-1600. This makes DDR3-1600 a very good "Sweet spot" in terms of price, performance, and not giving the user any stability issues.
If you have a set of Corsair Vengeance DDR3-1600 there's no need to replace it unless you're building a whole new computer and still want to use your old one. Given the current SDRAM prices this isn't a bad idea but I would not bother replacing it only for an extremely minor speed increase.
Lastly, most high speed memory modules also contain profiles for lower speeds as well. A module marketed at DDR3-1866 may also contain a profile for DDR3-1600 and will perform well at those speeds. You should be aware that while AMD's processors natively support DDR3-1866 this is only guaranteed under certain tight conditions; Intel is similar.
