it would be a good idea to set the current video card to use vesa drivers, so when you turn off machine, remove old card and put new one and boot the new one will find and use that same compatible driver
since this is a old nvidia card i'm not sure what driver it can use, probably a old one from synaptic, 173?
after adding the new driver or setting it if it is already installed don't forget to set resolution and the other parameters
if system refuses to load gui, from console you can configure the new card by writing XFconf and following the simple menu that will be displayed to configure xorg again