Nope, 512MB is not enough for winxp, even though u don't use applications that use virtual memory as a "must".
just to convince u, windows xp doesn't start to use virtual memory when u used up the 512mb, it probably look for pagefile way before that.
I tried to view a very badly created adobe file, my memory used up to 400MB.
Playing warcraft3 frozen throne, I switch out of a game and loaded interenet explorer, my memory usuage is 630mb.
right now i have 1gb of ram, i disable virtual memory, so far so good.
again it depends on what u want to run.