Not sure if AMD have changed how their drivers are removed when uninstalling from windows add/remove programs, but back when I had my 5870, this didn't remove the driver completely, as upon reboot, Windows would reinstall the driver I had just uninstalled. It was only the driver itself and no catalyst contral panel or anything, so it seems a copy of the core driver was retained by Windows (W7).
To remove this copy of the driver, I had to go into device manager and remove the driver manually, where there was an option to remove the driver completely. Furthermore, I had to do this a couple of times because after each reboot of the computer, Windows would install the previous copy of the driver it just removed, i.e. it kept copies of all Catalyst drivers I had installed since installing Windows. I knew all drivers had been removed when Windows only had the default Microsoft drivers to install which resulted in a low res desktop and no aero interface. This may have changed though recently with all the new Windows updates/service packs.
Either way, before removing the AMD card, you might want to boot back into Windows to see if it has installed your previously installed Catalyst drivers again and then remove it manually in device manager, over and over again until only the default Microsoft drivers remain. Not sure if this is really necessary, as Windows obviously won't reinstall the Catalyst driver when you boot up with the Nvidia card, but I did this anyway when going from a 5870 to 580 and I had no problems whatsoever.
