RDRAM is a proprietary format. It transmits at 800MHz on a very narrow 16-bit bus. This means that it is easier to put multiple channels on a system.
200MHz DDR (400MHz) RAM would have the same bandwidth as a Dual-Channel PC800 configuration and have noticably lower latency if it could be implemented properly. Along with a 400MHz EV6 bus protocol it would be a sweet setup.
PC2100 DDR (on the AMD760 for example) would transmit at effectively 266MHz with a 64-bit data width, giving it slightly lower throughput than the dual-channel DRDRAM but it does not need to be installed in pairs and it has a lower latency and slightly lower power consumption. Micron is working on PC2600 right now (333Mhz/166DDR) so we'll see...
Perhaps a server setup with a Dual CPU Athlon and a dual channel PC2100 DDR. That would be sweet and with 4DIMM slots per channel, it would be an easy and inexpensive way to allow for massive amounts of memory.
P4 still relies on the old GTL signaling protocol (albiet greatly enhanced) and while its a little easier to implement, the effect is that the Athlon will still have a greater bus throughput if it's EV6 is pushed to the max. Especially in dual CPU setups- Quad EV6 would whoop up on a Quad P4 bus. Sadly, I don't now if that will happen ever.
I like this recent trend of pushing bus speeds and memory speeds up instead of releasing faster CPUs. Its something that's long overdue. We had only grown our bus speeds less than 3x since the 486 processor (fastest 486 bus was 50MHz) while CPU speed has increased by probably about 100 times (first 486 chips were 16MHz and took 6 cycles to do some math).
Now that we've triple the bus speed again we have a little headroom to work with. Don't expect Intel to be happy with 400MHz. I suspect the move to the new P4 socket might go along with the introduction of PC900 or PC1000 DRDRAM or one of the QDR technologies. Maybe not- but its a possibility.
I wonder what bus AMD will use for Hammer??
Maybe they'll develop EV6+? *grin* 600MHz EV6.
QDR SDRAM is in the works too. That would go nicely with a 400MHz EV6 bus. 200MHz*4=800MHz.
Rambus would have to kick their memory up to 1.6GHz if they want to use the same format and keep up with 200MHzQDR memory. Imagine the pricetag on a few hundred MBs of that stuff!!!
*waiting to see the future*
Eric