you can look up how much it can take here: http://www.crucial.com/
select the manufacturor, and go from there. pretty easy, though my exact model didn't show up, it had the one below it, so I figured out that mine is able to take like 4 gigs. I'm almost sure that you upgraded to XP fomr an older windows, right? that may be your problem if so. yeah like peepnklown said, xp needs more than 256 mb/ram to run smooth. I only have 512, but compared to my old hp (128mb/ram), it really makes a difference.
What kinda pc are you thinking of upgrading to? If your not sure, I'm sure some of the guys at this place would be happy to help you pick!
tell me if that helps please
indestructable