when i install extra ram into xp do i hve to tell it that it is there. also how can i tell how much my computer will take.
What manages the RAM is your motherboard, not the OS. When you plug in the RAM stick the MB should automatically detect it.
How much your computer will "take" depends. If you have a 32-bit OS it can only handle 4GB of RAM, and as Windows bites 0.5GB, you have 3.5GB free to use. 64-bit systems do not have this limitation.
You might be trying to put more than 4GB memory into a 32-bit system and not getting it working. Try installing a 64-bit system instead.