Depends, but if you are trying to get RAM online I recommend Crucial.com
My machine hosts 320 MB ram (64+128+128) and it's sufficent for some multitasking (photoshop, icq, several other apps running same time) on windows 2000 Pro. If you are running windows 98 and you're not too harsh on your PC (opeing lota windows, using Netscape 6.1
) then 128 MB should be enough, 256 will get you almost anywhere.