Having just read a post here by Attache with best answer from Geofelt it seemed that this was the answer to my current problem.
I replaced my TWIN3X2048-1333C9DHX G (2 gig total - 2 sticks) with 8 gig using 4 sticks of TW3X4G1333C9A - both Corsair modules.
Vista 64 failed to boot correctly with a BSoD before it finished loading - as in Attache's post too fast to get details.
The system ran all the modules at 4 gig and all the banks and modules work fine (much swapping and gnashing of teeth).
I am currently running 2 sticks of the TW3X4G1333C9A and 2 sticks of the TWIN3X2048-1333C9DHX G for a total of 6 gig.
CPU-Z shows that all sticks are running at 1.5v (in the current 6 gig setup). My mobo is a Gigabyte GA-X38T.
Just wanting confirmation that uppping the voltage is often needed in this situation, and more interestingly, why, given that the board is sposed to support this amount of memory population in the first place !!! Never done any voltage changes before so just wanted to double check before tinkering in that area.
If you have a socket 775 board, and are trying to use more than two DIMMs, you will need to 'bump' the MCH voltage by a tenth, or a tenth-and-a-half, to accomodate them. Realize that, if you are doing this, and running memory faster than 800, which is already 'overvolted' (likely, 2.1V instead of 1.8V JEDEC), you are both over-volting and over-clocking your MCH - stability may require MCH cooling: