for Intel(R) Pentium(R) 4 CPUn@. GHz, I have only 768 MB of RAM - - - How much more memory can I add?
It depends how much your motherboard can handle and what OS you are running. It looks like you have a stick of 512MB and a stick of 256MB. So, you can at least have to sticks of memory. For a P4 I wouldn't put more then 2GB. An more and you'll be waiting money. Older DDR memory is so expensive now. I would recommend saving up for a new system.