It has been a while since I went through the process and I can't seem to remember how it went with Windows, but I'm under the impression that I uninstalled the previous GPU's drivers, booted with the new card to a black screen, logged in with Remote Desktop, to realise that Windows was installing nVidia's drivers through Windows Update. I later installed the latest ones downloaded from their website.
Similar to my advice to BaronVonBehemot, make sure you have Remote Desktop enabled, setup and test it before you change cards.
I believe you will not be able to run nVidia's driver setup without the card being installed, because it should hang, warning that "no compatible hardware has been found on this system", or something similar.
Also, depending on your card (and your luck or lack of it), you might need to modify a .inf file on your driver installer folder for the setup to run.
Are you considering selling your 5750 after you make the upgrade? I'm actually thinking of downgrading my 2011 iMac, as I ended up giving up on the 780M. I'm looking for a solution that will allow me to have the system running with an operational boot-screen and native brightness control, while avoiding the prone to failure AMD 6xxx series. If my reasoning is right, I guess my best option would be one of the AMD 5xxx series found on the previous generation 2010 iMacs.
Can anyone pitch in and confirm if my logic is correct? A GPU taken from a 2010 iMac should work natively on a 2011 one, right? The drivers are all there... am I missing something?