I don't think there is much I can add to this other than confirming that 1gb is under par for any thing these days, 2gb seems to just barely cut it. I found EZD w/DFH to be relatively decent on memory, it's EastWest's DFH 2 that was a huge pig on ram.
I'm going to assume that every one is using a non built in sound card because, any thing else would just be madness. Aside from more ram and disabling any non used programs (especially ones that idle in the user tray), that having a PCI-E video card will also improve performance though, I think even most pre-built PCs have these now a days.
As for VISTA, I use to run it with 2gb and just my DAW and plug-ins with out having any performance issues whatsoever, granted the PCs sole purpose was just for recording and, it was one of those nasty pre-builds from Wal-Mart (wasn't mine, I normally hate pre-builds).