dual channel tries to use both RAM disks evenly with their own individual bus, so they would theoretically give double the transfer speed
but like said before it usually doesn't, it's more like 150%
this is the same with SLI. SLI means 2 video cards processing the same screen, which would theoretically double the performance, but in reality is more like 150% as good
when you see a "400MHZ" DDR RAM disk, it is actually 200MHZ, but it transfers 2 times the data per clock cycle as SDRAM, giving performance like if SDRAM were at 400MHZ
RDRAM is similar, but can be "800MHZ" or "1066MHZ" which is actually 200MHZ x 4 or 266 x 4
RDRAM transfers 4 times the data per clock cycle than SDRAM
RDRAM is twice as fast as DDRRAM, but it never really took off. RDRAM was much too expensive for most people, and few motherboards were made to support it (not sure why)
socket 939 is the way to go, like said it has a 1000/2000MHZ HT over socket 754's and socket 940's 800/1600MHZ HTT
socket 754 doesn't support dual channel
socket 940 requires you to use ECC RAM, which is more stable, but slower and more expensive. it is most commonly used in servers (which sorta suits socket 940, since it holds the Opteron server processors)