Okay, last data point from me tonight -- on recent-vintage Sempron boxes for office usage, using Ubuntu's default Gnome environment and, notably, a 256MB VMWare VM for the legacy Windows image...
512MB RAM appeared livable, with modest grinding when loading OO.o. However, given memory leaks and all other contributing factors, after a month of uptime running a "standard" small-office stack (OO.o, a long-lived Firefox session averaging a dozen tabs, the Windows VMs) things would begin to get painful; specifically, trying to work on five or ten documents at once with OO.o would get even more painful.
Bringing the machines up to 1GB effectively eliminated the issues; any memory leaks don't outrace my haphazard patching schedules, and it's possible to juggle as many OO.o windows as one could want without getting into trouble.
This is, of course, a ridiculous amount of bloat (almost entirely on OO.o's part), but pragmatically, it lets work get done and leaves plenty of headroom, especially if one assumes OO.o will eventually see rounds of profiling and optimizing after the scramble for feature-completeness is over (in a decade, let's say; something similar happened to Mozilla).
In comparison, a PPC Mac Mini with an equal 1GB of RAM will still bog down when using NeoOffice from time to time... and you'll be wishing for far more than that if you get serious with Illustrator on that platform, these days.