Good question marcusmagic

and a difficult one to give a concrete answer, but I'll try my best:
RDRAM is more expensive, as the yields of production vs. SDRAM/DDR ram are lower, which means it costs a little more to make. Also RAMBUS the evil sue-happy company makes all builders pay a licensing royalty fees, which also adds a few bucks to the price.
RDRAM has a slightly higher latency than DDR ram. This is masked generally by the cache mechanism which is part of the architecture, so I haven't noticed any tasks in which this makes a difference yet.
The main advantages of RDRAM are the incredible memory bandwidth, and the serial interface. The way the P4 architecture is designed is with a much longer pipeline which will of course allow it to scale to insane GHz speeds. It's only <i>just</i> starting to flex it's full muscle at around the 1.8Ghz mark, higher clocked CPUs will really start to show the advantages of the P4 architecture. The serial interface (vs. the parallel interface, which involves insanely more traces to address the same amount of memory) is important, as you reach higher and higher clock speeds, because it involves less traces (trace=path printed on the circuit board (PCB)) to the now-HUGE amounts of available ram. This means that at higher clock speeds and ram amounts, it is easier to get a stable feed of data to/from the RAM (because less traces means less memory timing issues to get a stable system).
DDR memory has lower latency than RDRAM (but again, this isn't as important with a good caching mechanism), and when used in a "dual channel" configuration, you get very similar performance.
The long and short of it is, they should both perform very well with DDR being the cheaper solution, and RDRAM will become more of a niche product for 'high end workstations and servers' as time goes on-- unless RAMBUS the company opens up the licensing a bit -- or for some reason the amount of traces required by DDR make it unfeasible for higher clock speeds.
I guess I'll sign this point...
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<font size=-1>[ This Message was edited by: subhuman on 2001-11-29 16:51 ]</font>