More RAM usually means having more programs open. Check task manager (or however it is called on linux) and see how much RAM you are using. If it's 80% or more, it MAY help, but it won't be world changing. If it's less than that, I wouldn't bother.
It uses DDR2 odds are it is just old though adding more RAM will help. It may also help to switch to a lighter DE mint comes default with Cinnamon which isn't the lightest around (not the heaviest either though). That should lighten the CPU load some.