This will all depend on your motherboard and what is compatible with it.
If you want to upgrade there are two options, the first is to add more, the second is to replace with better. What you do would all depend on your motherboard.
If you dont know your motherboard: Download this little program CPU-Z
The infomation you will need us under the Mainboard tab, you will need to know the Manufacturer and Model number. Out of interest also post what RAm you have now to see where any improvements can be made. (use the same program under Memory tab)
Post back and I'll tell you exactly what is compatible
Since your PC is quite old, it's going to be DDR, or SD RAM, these are quite old. But it doesn't end there, you need to find out what is compatible with your motherboard, and how much you can actually place onto the board. If you open your PC tower, look directly in the middle of the board, manufactors usually place the model name of the board there, if you can't find it, look. Google your Board, research, and see what RAM freq you will be able to place on the board, thousands of people, just buy any type of DDR, DDR2, DDR3, and not check out the freq for the board, this is hugley incorrect since it could do huge harm to your motherboard.