There is no speed difference between 1 and 2 DIMM configurations as long as you use matched DIMMs (same specs for each DIMM) in DIMM pairs. If you buy it with 2 DIMMs, the manufacturer will use matched memory, so there's no need to worry. If you add a second DIMM yourself later, just make sure it's the same type as the first DIMM-- e.g. DDR3 1333MHz in the case of the X220.