Both currently available versions of Rambus and DDR have basic clock rate per bit transfer limitations (i.e. 800 Mb/s per data pin RDRAM, 333 Mb/s per pin DDR). There is an architectural limitation for DDR which was addressed by Rambus by using controlled impedance and a serial connection for modules, whereas DDR is an extension of SDRAM where you have a parallel bus structure with modules acting as un-terminated 'open stubs' on the bus (potentially resulting in signal integrity problems at high speed, such as signal reflections).

A serial bus, by electrical limitations or laws of physics, can sustain a higher bit rate per data pin than a parallel bus. Why is this? Because it has controlled impedance, no open stubs, and proper transmission line termination on the bus. This is part of the reason that you have to use continuity RIMM in empty RIMM slots to ensure reliable performance.

In order to overcome some of the limitations, DDR circumvented this data transfer rate restriction this data transfer rate limitation through the use of a wide, 64-bit bus, whereas Rambus only uses a 16-bit bus. Of course this has led to some additional problems with signal integrity issues when multiple sticks of DDR are used on certain motherboards. More module sticks on the parallel bus result in more signal integrity issues that need to be resolved. We have even seen this in the debate over the nForce Superstabilty mode. Although not as well known the Tyan Trinity 510 also suffers from some of these issues as well and is forced to throttle down the speed of it's DDR as well when multiple modules are used.

The reason for this, above all, is that at higher clock speeds, RDRAM is the only type of memory that can make use of the increased bandwidth of the fastest CPUs. Still, RDRAM has had to fight against a big problem from the start in order to make it in the mass market: the price for the module is higher than that of SDRAM. Much has been made of the price vs. performance ratio of RDRAM, but the truth is that RDRAM is expensive to produce and cannot be produced or tested on the current equipment that most DRAM companies currently have. Of course if these memory makers are going to have to invest millions of dollars in order to upgrade their equipment, some of this cost is going to be passed on to the consumer, which in part leads to the current higher cost of RDRAM.