Download the new graphics drivers and save them to a place you can easily locate later
Remove the existing graphics drivers
Shut the computer off and unplug it
Open the case
Remove the old graphics card
Install the new graphics card
Plug the computer back in and start it
Once you're back in the OS, (the screen will look strange, because you're running generic drivers and don't have the resulotion set any more) - locate the drivers you downloaded in step 1 and install them.
Profit