Dual DDR channels. There are two benefits, 1) Twice the memory bandwidth
(obviously) 2) More memory (4 DIMMs rather than 3). Also because each
channel only has two sockets the system memory system is more robust. On
754s there are 3 sockets on the single channel, populating all three
sockets with PC3200 DIMMs is going to be touch and go, on the 939s there
are only two loads on each channel so populating both sockets on each
channel will be much less problematic.