I think there is enough information in here to answer the original question so I'll close this before an argument gets developing...
you can have as much ram as you like, (assuming your board can handle it)
whenever you buy a board it'll say how much RAM it can handle.
Look up on the manufacturers website how much ram your OS can cope with, (as said here XP has a limit)
if you want a lot of ram then you have to buy big sticks and a board with lots of slots.
as a side note, a lot of big ram boards out there will have been intended for server use.
for examlpe this board.
says it supports 8GB of RAM
(that's 8 2GB sticks.
whilst this one only supports 2GB max.
that could be 2 1GB sticks or 1 2GB stick.
so yeah, don't just go and buy a load of ram and hope for the best, make sure you have a board that will support that much RAM first!!