|For XP, 512Mb is just enough to plug along. 1Gb is pretty good as long as you don't have a lot of stuff doin' at once.|
About every second website you visit is streaming video in your face now, nearly every browser is tabbed and few folks have only one tab or window open.
So 2Gb can probably go down the tubes pretty quickly too; just depends what you're doing.
You motherboard apparently does dual channel..that's why 2 X 1Gb sticks (matched sticks) of memory are recommended. I imagine you now have 2 X 256Mb installed.