Mushkin.com explains it:
What is the difference between dual and single bank?
DIMM modules come in two different configurations, dual and single bank. Whether a module is dual bank or single bank depends on how the chips on the module are configured.
Don't confuse the terms "double-sided" and "dual-banked". Double-sided is a physical term meaning that chips are arranged on two sides of the memory module. Dual-banked is an electrical term meaning that the module is divided electrically into two memory banks.
Why do I need to know if a module is dual or single bank?
The amount of memory supported by a motherboard is determined by the number of banks of memory your motherboards chipset supports, most people mistakenly think it's limited by the number of modules your motherboard can hold. For example even if a motherboard has three module slots, it will not accept three dual bank modules if the chipset is limited to 4 banks of memory (three dual bank modules equals 6 banks). At most it will only accept three single bank modules (1+1+1 = 3), or one dual bank module and two single bank modules (2+1+1 = 4).
