Your spec should be entirely adequate as it is. I think it would be a mistake to replace it without finding what is zapping all that RAM.
I have a friend who uses a slower CPU, same RAM (which is maximum for his Win98SE) and has no bother with some huge Photoshop files. ..... He doesn't have an internet connection. I think there is another reason to see what you can do to avoid running unnecesary stuff in the background.