XP doesn't have the problem with memory that 98 had. In the latest Mazimum PC magazine they said they found the largest gain in performance (unless we're talking working with graphics programs) with XP to be at 512 meg. I don't know if they know what they're talking about but there you are. FWIW, I have 512 installed on my machine running XP. Runs nicely.
ME is just a dumbed down version of 98SE, and as such has the same limitations on ram i would think. Personally, i've never run into the 512 meg problem in 98SE, i have had 1028 meg for a long while and never have had any trouble caused because of it.