And 64 Mb is ok for Windows 2000, probably for XP too.
You have a strange definition of "ok", methinks .
I had 512 MB ram with WinXp and while it certainly worked, having to use freeware programs to free up as much RAM as possible in between games was a real hassle, not to mention the massive swap file writing to the harddrive whenever I tabbed out of a game. After I installed 1.5GB in that PC, it was like entering heaven itself, the difference was unbelievable. I don't think I'd dare try it with 64MB
In my more modern computer, the jump from 2GB to 4GB (well, 3.4'ish with 32 bit XP) was awesome as well. From 4GB to 8 was good too, but nothing as noticable as the previous two.
It puzzles me a bit how games seem to demand increasingly more powerful graphics cards that cost oodles of money, but no one jumps on the 64bit wagon and shows us what they can do with 6-8GB of RAM. Can't we just leave 32 bit to die in a ditch already, please?