Dual channel DDR sounds to be about right. The thing I'm wondering, though, is it enough? Am I correct in assuming the PPC 970 has a 64-bit bus? If so, then even with dual channel ddr400, it would still be short of the full performance of the bus (on chips higher than 1.6ghz). I really don't see much of a way around that at the moment short of some odd solutions -- maybe some sort of high-speed serial memory like rdram, or possibly even quad-channel ddr. I would imagine that going with dual ddr400 wouldn't be too much of a performance hit at 1.8+, but I would think that apple would want to switch to DDR2 as soon as they could, especially with higher clocks in the works.
As for the second bank of memory slots, just a wild guess... I wonder if maybe each processor will have it's own bank of memory (a la Opteron). I know Opteron has it's own memory controller, but maybe the 970 will have a dedicated "north bridge" for each processor in a DP system (sorry for the PC term, but I'm not sure what it's called in the Apple world -- I'm a recent switcher
). It's possible that maybe they'd use the same basic board for both single and dual processor machines, and that they just put the sockets on the prototype, while there would just be empty solder pads on a production single processor board. Having not seen it for myself, that's just wild speculation, so take it with several large grains of salt
Edit: Just looked back at the specs for the 1.8ghz chip in that IBM pdf, looks like dual ddr probably would be enough, at least in the short term, assuming Apple doesn't go any higher than 1.8 before the end of the year. Hopefully by then ddr2 should be available and should facilitate the higher clocked 970s. In all, I'm pretty anxious to see what one of these puppies could do on Divx encoding, which is what I use my PC for primarily.
-Leigh
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