I am kind of surprised that no one actually bring this issue up, cause i think it is rather important.
if you are using laptop, the big difference is the POWER CONSUMPTION.
according to Kingston website:
a 2G DDR2 SODIMM need 1.944W during operation.
one 1G DDR2 SODIMM need 2.6W during operation. so dual channel means 5.2W for 2GB
that's a 3.3W difference!!
I think you will see AT LEAST 40 minutes of battery life increase if you choose ONE 2GB over TWO 1GB.
btw, Dual channel was really beneficial in the old Pentium 4 CPU. on the Core CPU, run single channel and dual channel will make very little to no difference in real life performance.
by comparison, a 7200rpm HD need around 2.8W during operation. a 5400rpm HD need around 2W during operation. the difference is not that huge comparing to 3.3W difference in RAM.