I am wanting to add some more RAM to my computer. But I don’t know what to get. Mine is a custom built computer so I cant go by brand and model. I do however have the owners manual for my mother board.

This is the information listed;
FOXCONN 865A01 User manual. four 184-pin slots with DDR DIMM for installation of DDR 400/333/226 memory bank.

The computer that I want to add RAM to has never been hooked to the internet and has no virus or firewall software so internet diagnostic is not possible. That computer has been dedicated to at home gaming and personal projects. I want to get it upgraded and ready to hook up to internet to play WoW.

I looked inside, and I am only using two of the four slots. I remember them telling me when the system was built, it was shared RAM. But the original RAM was only 256MB and that went bad after about a year and they replaced it with 512MB RAM So that’s what I have, a total of 512MB of ram. I want to upgrade up to a Gigabyte.

Couple of questions; Can I just add a couple more memory banks? Does it matter if they are the same brand or not? Do I have to replace the ones I have and get all new when I do the upgrade. Does RAM have some kind of driver like video cards or is it some thing the computer just automatically recognizes and uses? which do I have the 400/333/226? If I can just add to what I have, what should it be?
Mother boards max RAM is 1G & my operating system is XP.