98 & me should be able to use over 512MB ram, most likely the limiter is your hardware. Your motherboard will have 2, 3 or 4 SDRAM slots, and each slot will be able to address a maximum of 128, 256, 512 or 1024 Megs, as it stands right now. That varies greatly between motherboards, but to answer your question, XP can address 4GB or RAM, if I remember reading that correctly.
/L.A