What you can do is hook up the old machine to a external monitor and with Ethernet network conneciton between the machines use Apple's Remote Desktop to control the older Mac from the newer.
What you can do with this is run output things like iTunes to a stereo, heavy CPU rendering programs etc., using the old Mac's processors without tying up the processors on your new Mac.
The ol Mac graphics card only will render to the external monitor, and the image of the other Mac's screen is rendered by the newer Mac's video card. So there isn't any performance gain in graphics.
So this could be good for multi-tasking and for programs you can run on the old Mac's processors.