the "norm" just keeps going up and up
XP is possible in 64Mb (but not good)
256 - 512 is good for a business machine
256 - 1Gb is more than enough for home use
over 1Gb is extravigant for anyone but George Lucas or pixar.
part of the misconception is that windows will take as much as you can give it.
Load a prog ... and it will stay in memory untill something else needs it (or you switch off) - just in case ... the memory isn't actually in use ... it's just holding on to a closed prog in case you decide to open it again.
(if you do ... it will open in a flash ... try it with word and see!)
to most memory progs ... it's shown as "used" (it's also how memory managers "recover" so much memory when they run - it's a safe bet that most machines will have a min of 10% just waiting ...if).
2 things XP is quite good at ... is using a registry full of old rubbish without problem ... and memory management.
however ... you've got to remember that XP will create a swapfile 1-2 times your total RAM - however much you have.
so put too much in and the relatively slow speed of your hard disc will eat any advantage ... plus some .. so your machine can actually slow down if you just bung it loads of memory without reason.
the real advantage of excessive amounts of memory is you get to hang round the playground or pub impressing the muppets with the size of your ..... ram
if you game ... or render 3d landscapes ... more than 1Gb is useful - if not ....
(vista is different ... though)