There are two banks on a double sided DIMM. If you had 2 dimms that would give you 4 banks. Basically, each time you have to "bridge" a bank it increases latency which is bad. So, the higher density, the faster it will be. The difference will be completely negligable though and you won't notice any real world difference. In memory intensive applications, yes, there is a difference between DDR266, 333, 400. Hope that helps.