I find myself in a similar predicament, except instead of sudden power-down the computer randomly freezes (sometimes not so randomly, I can't use Excel anymore). Windows and drivers are all up-to-date and virus free so it's time to start swapping hardware (if I had it to swap). I would say you are probably at that point too if both of your memory sticks test ok.
I could have sworn I read a more recent write-up of this "issue", but
here and
here are a couple articles I found from a quick search that might explain why you can't see all 4GB if you're not using a 64bit version of Windows.