400MHz DDR memory on a single channel is 3.2GBps. A P4 with a 533MHz bus is 4.2GBps, which means the memory bandwidth is not sufficient to keep the bus constantly full.
An Intel RDRAM chipset uses two channels of RDRAM, at 1066MHz (8X-data rate) to achieve 4.2GBps, matched with the P4 bus. (A 400MHz bus chipset would use PC800 RDRAM).
The important factor when you compare these is obviously that the single channel of DDR memory at 400MHz comes close to matching the P4, while the fastest speed RDRAM requires two channels to match the P4's bus. RDRAM fanatics fail to acknowledge that the only reason RDRAM ever had any chance at competing with DDR memory was due to Intel having created a dual-channel chipset. A single channel of PC1066 RDRAM only has a data rate equivalent to a single channel of DDR266 memory. RDRAM is only 16 bits wide, while SDRAM is 64 bits wide; RDRAM has to have a clock rate (or data rate multiplier) 4 times that of SDRAM to get the same bandwidth.
However, a dual channel DDR chipset was harder to create, because the wider data path makes signal integrity harder to maintain (and a lot of other technical stuff that I can't explain because I'm not an electrical engineer).
The Intel Granite Bay chipset, and SiS's upcoming 655 chipset (and one VIA is working on) use dual-channel DDR memory. With two channels, DDR memory can operate at only 266MHz, to achieve an effective 533MHz rate of 4.2GBps, same as the P4 bus. Since DDR is already at 333MHz and 400MHz is soon to become a standard, while RDRAM is only just keeping up, the DDR frequency can easily be increased to keep up with the P4 as well; it already has a future path. The Intel chipset doesn't let you run the memory at 333MHz with the bus at 533MHz, however the SiS chipset will, so the memory bandwidth will actually exceed the requirements of the P4 bus, until Intel starts using the 800MHz bus, by which time who knows what will be happening with memory technology.
I forgot too, RDRAM originally was going to be widely available at several speeds, with 800MHz being the best. 600MHz was also going to be available, that being equal to only 1200MBps, slightly better than PC133. RDRAM was barely competitive when it originally came out, due to higher latencies than SDRAM and not much need for the higher bandwidth at the time. DDR SDRAM would have annihilated the RDRAM market if not for dual channel RDRAM chipsets.
Even a "32 bit" RDRAM motherboard like the Asus P4T533 is still technically using a dual channel chipset; they've just mounted chips onto a single module so they're cheaper to make, and cheaper for the components on the motherboard. Still is two separate channels, they just go to the same physical module.
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