I think you might refer to the amount of memory. The type of memory that goes in it depends on the mobo's specs. You will not be able to change the type of memory that goes in there, as different DDR has different specs from DDr2 and so on.
So, pretty much a mobo will accept one type of memory and that's it.
In order to take advantage of the 64bit OS you will need at least 4GB of memory, whatever type of it accepted by the mobo. The 64 bit OS will use it more effectively if more memory is available, so in this case more is better. A 32 bit OS can not fully address even 4 GB, so no need for more than that.