DDR3 is out already, mainly used for RAM on vid cards and is not used (currently) for system memory.
running lowest common denominator RAM timings doesn't necessarily mean they'll run in dual channel. some ICs prefer to be run at certain timings and voltage and lowering latency (or increasing latency for that matter) doesn't mean they'll run in dual channel succesfully. most of the times they will and, not trying to scare anyone, but it's not guaranteed. if you don't know what i mean, read up on RAM latency and mobo compatibility (per IC).
frequency and capacity are completely different things. if you don't understand those two mutually exclusive concepts, well...read up.
socket 939 is far from dead (most likely at least 2 years away)...don't let AM2 scare you away from building a 939 system.
as i already mentioned, AMD's move to DDR2 is primarily for dual/multi core systems.