1) 1GB vs 1.5GB vs 2GB vs 3GB vs 4GB RAM?
do not ever use 1gb ram with vista it will make you life incredibly miserable. computer behaves like a daihatsu carrying 5 sumo wrestlers. surprisingly 1.5G acted like a turbo charger and there is actually no improvement between 1.5 and 2 G regardless of dual channel or not IF you're doing normal things like surfing net and playing video. boot time is minimum with 1.5-2G (58 sec). the next step of improvement comes at 3GB - doing pretty serious dvd video editing, running photoshop elements 9 whilst opening 10 windows in safari 3 and firefox 3 and ie7. increasing to 4GB of ram had minimal user impact.
conclusion: 2GB dual channel is best for most people. 3GB is max for most people who dont do hard core multitasking/video tasks. surprisingly boot times are slower with more than 2GB ram.
2) vista does not use all the ram it can?
the laptop has very few applications to load. with 3GB ram after booting, 860MB ram is used up. with 2GB ram after booting 650MB ram is used up. consistently. why?
the desktop has a few more apps to load. with 4GB ram after booting 1150MB ram is used up. with 2GB ram after booting 860MB ram is used up. sometimes its 950MB. again ... why?
3) dual channel vs single channel.
no difference if you dont max out your ram bandwidth doing high ram bandwidth things like ... err obviously i dont do them. probably does makes a difference actually as your WEI goes from 4.6 to 5.9 just by dual channeling, but it is not felt in normal use like booting up, playing media and surfing.
4) ddr2-667, 800, 1066?
no idea! i didnt feel any difference playing games and certainly makes no difference in booting times. probably the brand and therefore timings are more important than the ddr value - then again probably only makes the difference in hardcore users.
google says that nowadays ram is not the bottle neck, as long as you have enough. so just make sure you have at least 1.5GB if you're a normal person running vista. find the bottlenecks and fix them - graphics card for gamers, bloatware for all branded computers, malware for the unwary, ETC