I've got an old desktop that I'm keeping alive to act as a fileserver. I've always just used the integrated video (VGA-only), which has always been good enough, since I'm not running anything 3D.

So in theory PCI-e is backwards compatible, i.e. that if it fits physically, it'll run electronically. In reality, we hear stories all the time about the x16 slots not working for anything other than ...