Intel has published a list of tested memory, that can be found at http://support.intel.com/support/motherboards/desktop/al440lx/al_mem.htm.
A lot of people have asked if they can use PC100 or PC133 memory with this motherboard. The answer, thankfully, is yes! You can also mix different memory speeds -- just realize that no matter what type it is, it will only run at PC66 speeds. I have run PC66, PC100, and PC133 in my machine, all at the same time, with no problems. The most important thing to look for when buying memory for this motherboard is to make sure that it has the EEPROM for SPD (Serial Presence Detect).