I spent too much of yesterday fiddling (for family reasons) about with trying to place a new 512Mb SDRAM in an old mobo running 2K. There were two slots and a choice between 2 older 125MB chips.
Eventually got it right by trial and error, and some of the errors were frightening! .......... In the case of this mobo I had to seat the correct size in the correct slot. It didn't help matters to find out that one of the old sticks was PC100 while the others were correctly PC133.
AS crx1600 has said - earlier versions of Windows find too many MBs too rich. 98SE would not boot for me a while ago when I tried going over 512Mb.