XP runs fast on 1gb plus....with all updates......
256mb was enough for my folk's Athlon XP 2600 - it ran fast..very fast..., no additional software, but as soon as the 'fixes' came in, it was slow..........
Another 1 gb was fine.
My system is about 3 years old - Athlon 64 3000 with a good video card, and 2 gb - physical limit on performance 2 gb max, 3 gb would have lowered it - board limits....
New machines have 4 gb plus as a minimum, and higher performance ram...lots of geeky stuff to digest, etc.....