I just skimmed through the thread so I don't know if it was brought to your attention yet, but all 32-bit versions of Vista support 4GB ram now. I have not tried more, but I know 4GB works.
Just install Vista SP1. It is not on the auto update yet, it will be in about 2-3 weeks. It's on microsofts site as a 450ish MB download if you want to manually install it right now.
My P4 machine with vista didnt change much, but my Core 2 Duo machine runs much better now.