Pairs...matching pairs, same brand, model, size, etc. With A64 motherboards, can use memory/cpu dividers to run the slower memory. Since the memory controller is on the cpu, this works well. Sure can add lifetime/use to them ole PC2100 sticks. For example, if you have 2 sticks of matching pairs of PC2100 from a system you built 2 years ago, it should work fine. If they would work in dual channel mem config on an older motherboard, they should work on the new 939s.