Think of DDR3-1333 RAM as higher quality DDR3-1066 RAM.
In fact, that's what it really is. RAM comes off of the production line. Each batch is to determine the overall quality. If it is "average" then it batch is labelled as DDR3-1066. If the test shows that the batch is "higher quality", then they can be labelled as DDR3-1333 or faster RAM.
Overall, installing faster RAM just means that they will be running at lower speed unless the BIOS gives you the option to set the RAM speed above DDR3-1066.