I'm absolutely shocked that 98 is leading the pack right now. I've used Win2k flawlessly on systems as low as Pentium 133. That would be your best bet. If it was >=500Mhz, WinXP would be the one to go with but you'd need some more RAM as well.
Whatever you do, just don't use 98 or Me!!! There is no point to using either of these OSes anymore. 98 is decent but still very old. Me is arguably the worst OS M$ ever released.
If your system can't handle at least Win2k, get a new system.
