Are you talking about buffered (registered) ram or the HDD's cache?
the former: buffered ram is typically used on servers. There is a register that stores the information taken from ram for a clock cycle, which somehow offloads some work from the memory controller and allows more memory to be installed in a system.
the latter: from what I understand, the HDD cache is a middleman between the data on the hard drive and the drive interface (IDE, SATA, etc...). Higher cache amounts translates to a longer burst transfer* (burst transfer is much faster than sustained transfer...ideally around 300MB/s for SATA 3GBps burst and around 70MB/s sustained)
*this is if my assumption about how cache is actually used is correct. I assume the the buffer is first completely filled before sending data to the mobo