The latency of RAMBUS memory is not that high.. the inherent latency is quite low..but the fact is, RDRAM has 16bit bus, but Intel chips since the Pentium has 64bit address for their MCH.. so the data from memory has to get rearranged in MCH.. that's what causes the apparent latency probs in i820 and i840. In other words, you can't say RDRAM has high latency, but it is true that the memory subsystem in i820 and i840 has high latency compared to SDRAM-configured setup..(SDRAM has 64-bit bus).
In P4+i850 setup though, L1 and L2 caches are designed to hide this problem; They have ulra-low latency. eventhough i850 has the same 16bit->64bit problem, because of the caches being so fast, the latency problem is not that apparent. Dual channel setup helps a lot too.
In actual tests, P4+i850's memory subsystem has about the same latency as P3+PC133 SDRAM(with supporting chipset).. which is quite an improvement. and yes, with DDR SDRAM, the latency will be even lower, a lot lower than AMD TB+AMD760+DDR SDRAM setup. This can make P4+DDR SDRAM+Brookdale DDR faster in some programs that doesn't require a lot of bandwidth. This is something we should wait and see.
so why does RDRAM use 16bit bus, you might ask.. because, in SDRAM, with its 64bit bus, it is hard to raise clock speed.. as the clock gets higher, there are more trash data being generated in the system, resulting in lost efficiency. This problem gets more serious as clock speed gets higher. So, theoretically, it will be easier to make it clock higher if RDRAM were made to have 1-bit bus, but since Intel processors(also AMD processors)have some 16bit microcodes that control other parts of the computer, it is most effcient to make the bus in 16-bit form. RDRAM would lose little efficiency compared to SDRAM as clock rate goes higher, because the data chunks coming in and out of the memory are a lot smaller.
Anyway, plz don't say "RDRAM has High Latency" thing because it is NOT true. it depends on how you implement it.