Very often I speak with people who say that they have upgraded to a new Mac, either because of the new operating system or speed, but that there was nothing actually wrong with the old one (beside being old)!
This doesn’t mean that that this should be the end of its life. A while ago I came across a new app called ScreenRecycler that gave my old machine a new purpose in life. Your computer doesn’t even have to be a Mac, it could be the old PC that you upgraded. This app is a commercial app but it’s worth every penny (cent) at $29.90.
It’s very simple to use, after you have downloaded the application install the ScreenRecycler on your new Mac, then on the machine that you are going to use as a monitor, install JollysFastVNC ScreenRecycler and away you go.