Using 4GB of RAM with 32bit Windows XP has some issues click here
Do a bit of Background reading
For VISTA and 64bit versions of Windows 4GB might have an advantage.........it really depends on what you are doing with the pc.
For gaming and general usage 2GB is plenty good enough.Heavy duty video editing and CAD would benefit from more.
I would go with 2GB to start off with,but make sure it is 2x1GB sticks leaving you room for adding additional memory later.