Your title doesn't really reflect the question you are asking but here is some advice any way. I would go to the Crucial website and run the scanner - this will confirm the number of slots your motherboard has for RAM, the maximum your motherboard will support and the correct type of RAM needed. I would then look at OFFTEK to purchase the RAM from.
There is no point in going to 4GB unless you intend to install a 64 bit OS. Windows XP is a 32 bit OS and can only utilise about 3.5GB RAM
Before you start I would download and save, Service Pack 3 for XP, and an anti virus app.