If a motherboard doesn’t explicitly support a RAM frequency, is still compatible

Both memory modules should work and operate at 1066 MHz. If properly supported, the modules will operate in dual-channel mode, but both modules will be limited to the slower one, which is unlikely to be noticeable. There should not be any significant problems in terms of system performance or stability. You can consider using Memtest86+ to check for problems after you install the new memory module.

Edit: You stated that your motherboard supports 1066 MHz in single channel mode only. You can still use the memory upgrade, but you will not obtain any performance improvements related to dual-channel operation.

I'd like to comment on the RAM frequency (1333/1600/etc) part. Generally, the best stick is the one that has the ideal combination of:

• lowest timings
• highest frequency
• lowest voltage
• lowest price
• being compatible with your motherboard.

But the first 3 factors are not set in stone. For example, if for the same price, you can get:

1. a stick of 1333mhz ram rated at 9-9-9-9 at 1.5V
2. a stick of 1600mhz ram rated at 9-9-9-9 at 1.5V

Stick #2 is the better stick here. Because if you "slow it down" to 1333mhz, you may be able to run it at better timings such as 8-8-8-8, or at 9-9-9-9 with a lower voltage, 1.4V perhaps, or just run it at 1333mhz and call it a day. They're practically the same chips, just tested to perform at the stated minimum specs. In other words, don't give up a good sale because it's a 1600mhz stick!

Compatibility is not set in stone either! If a 1600/9-9-9-9 stick doesn't run at this speed on a motherboard, it may actually run fine at 1333/9-9-9-9. Just like the 1333 stick of the same brand would. Of course avoid any stick you know beforehand may not be compatible.

And that is why most RAM default to 1333mhz in the BIOS: for best compatibility. It's often up to the user to configure it optimally (higher frequency, lower timings, or lower voltage) as per the rated specs, if he so desires.

Example

You can use CPU-Z to figure out the rated specs at different frequencies. Below are the specs for my ram module, officially rated 1600mhz, CL-9-9-9-9-24, 1.6V. This JEDEC table is embedded of the RAM chip itself.

As you can see, the official specs match the column for 1600mhz (actually 800mhz, remember DDR stands for double data rate). If I were to run the ram at 1333mhz (666), I could safely set the BIOS to run the RAM at 1.5V instead - in fact I should since anymore is wasted heat. At around 1200mhz, I could safely lower the timings to 8-8-8-8-22.

Now you may ask what timings could this particular ram achieve at 1333mhz and 1.6V? Unfortunately, that falls in the realm of the unknown (or the overclocking). In this case, it would be much safer to buy a chip that guarantees 1333mhz, 8-8-8-8-24 at 1.5V or 1.6V.