I disagree...for Win9x, 64MB is good and 128 is nice and roomy for the memory leaks caused by all the poorly-written programs out there. (THIS MEANS FOR WIN9X, 128 IS ALL THAT YOU NEED).
For WinMe, since they have a few stupid background utilities running which may help you out (restore registry), I recommend the 256MB of RAM. Again, 128MB is great of these machines, but since RAM is so damn cheap, might as well get 256 for those sytsems.
WinNT and Win2000, I would recommend ATLEAST 128MB of RAM. NOTHING OVER 512MB...I see real no performance difference on my Win2K box running with 256MB or 512MB of RAM, but again, since it was cheap, I got 512.
Bascially, the general rule should be, 256MB is where you'll stop noticing a difference in performance gains, and although very debatable, I believe that anything over 384--512 (depending on your machine) will probably hinder you a little more than it helps you.