That notation means that the CPU officially supports 1066 and 1333 memory, but you can force it to run faster by changing BIOS settings. Depending on which mainboard this is, it may have many speeds you can use or only a couple.
For instance, Intel Sandy Bridge systems can use 1066, 1333, 1600, 1866, and 2133 memory but nothing in between those. If you bought 2000 RAM, it would run at 1866.
If you are going to overclock on an AMD system, I suggest getting fast RAM like the 2000. If you buy 1333, you may be limiting your overclock.