Upgrading my computer?

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Ok, my computer is kind of old, however when I got it it was top of the line. I am running XP. I don't want to get a new computer, rather just upgrade this computer. Firstly, I only have 80GBs of memory. I need more, as I do have a lot of stuff on that computer, and a lot more which is just stored on disks which I want to put back on. What would you suggest? External hard drive? I only really want an extra 100 or so GBs. How much would that cost me? Secondly, I would like more RAM, mainly for gaming purposes. I have only 512MBs, and I would much prefer to have 1 GB. If I buy RAM that plugs into the expansion board, will I need to buy a 1 GB, or can I buy 512MBs and have them "merge"? Also, I don't know if my RAM is PC1, PC2 or PC3 (is it PC_?) How can I find that out? Thirdly, I am interested about the Processes tab of the windows task manager. I always look at memory usage, and is there any way I can get more...memory... to use? Is that part of CPU? Is that "virtual memory" which I have heard about when my computer is being irritating? Lastly, I don't know what my computer means by Raedon 9600 series. When I start a game in Steam, it says that is my video driver, and that I should upgrade. I would quite like to do this but I have no idea what any of it means, and when I click the link it says it has moved. If you can answer any of these questions, please reply