There is a stock article which will be making computer journalists dosh until well after the introduction of the matchbox-sized computer. It is always called '10 Things To Do With your Old Computer'.
Don't bother reading it. There is just one thing to do with an old computer. Creep out at night with it and find a skip.
OK, illegal, incitement to commit, etc. So, just get rid of it. In an environmental way.
Print servers? Linux firewalls?
Music centres? These three always turn out to be some of the 10 things. And it's only deeply committed geeks who ever get around to doing even one of them.
But, stop right there.
There is another option.
Sort of. Providing you haven't dumped the old screen. What you do is plug the old screen into the second port on your new graphics card, fiddle around with the right button anywhere on your desktop, click 'Properties', then 'Settings' and you should see the image of a second screen - which you adjust using 'Advanced'. Sit back and discover why twoscreeners are such a happy bunch. You may still have to creep out with the redundant box - unless you think you can cope with four screens.