honestly, you might be better off buying a new computer than retrofitting your current one.
Prices have gone down so much on new computers that you will actually save money. If you were to purchase the components you need individually, by the time your done paying for all of the shipping and taxes (if applicable) you will prob. pay more.
If money is tight, the very least you can do is upgrade your memory but since it is an older machine, you might be limited since the memory might not be made anymore.