If so, Yes. AM2+ and AM3 CPUs fully support DDR2-1066 memory. (Mind you, the X2 550 is an AM3 CPU, not AM2+ CPU.) It's the older AM2 CPUs (Athlon X2's) which do not support 1066 speed. They can only run up to DDR2-800 memory speed.
Just make sure you're purchasing an AM2+ socketed motherboard to go with DDR2 memory, or an AM3 board to go with DDR3 memory.
Oh, and you may want to double-check the manufacturer's website for the board's CPU support list. Many boards will require a BIOS update for full support of new CPUs.
The short answer is yes. However after you pick which motherboard to use with the PhenomII 940 make sure you visit the manufacturers site and make sure it supports both the CPU and the RAM. It never hurts to take the few minutes to check.