Although you tell us your motherboard, we need a little more information about it. Model number, etc...
If you click here download and install it will tell us the info we really need to know.
Also there isnt a 4 gig chip the highest is a 3.06 gig chip. Which i do not think you could have anyway, unless you renew the motherboard.
Generally, more RAM the better, dosnt matter how much RAM to a certain chip.
If you want to upgrade to windows XP you have to be certain that Windows XP will install on your computer without any problem. For example your graphics card my not be compatiable. If you click here it will run a quick scan to determine if your machine is capable of running windows XP. However if you click here and download the "Windows XP Upgrade Advisor" it will perform a more thorough scan of you system. It is a large download, you can also get this of cover disc on magazines.