check your mobo first
Check the chipset spec. The other chipsets may only support up to DDR2-667. If you install DDR2-1066, you may still only get DDR2-667.
Most recent chipsets should support DDR2-1066 without any problem.
If you mix them, BIOS will automatically default to the slowest speed.
DDR2-1066 is definitely worth the money if you overclock your CPU.