RAM, Windows 98 vs Windows 2000

I have been hearing Windows 2000 handles RAM more efficiently than Windows 98. I can not locate documentation other than Microsoft docs. I need to know how much RAM Win98 will efficiently use. I know Win2k will use over 2GB of RAM. I have a client running CAD software who is hesitant to switch to Windows 2000 from Win98. They have 256MB on a P!!! 800 system. I believe they will see a preformance increase with Windows 2000, can anyone PLEASE offer some assistance in supporting my opinion?