Are you able to play non Steam games ?
Regarding the fixes you tried with Steam. Is there a System Restore point that comes just after installing
the new gpu but before you had problems with Steam that you can try restoring to ?
Or, is there a Restore point for you Steam folder you can try ? ( R/click the Steam folder > Properties > Previous Versions ) .
I don't know what changing brands of gpu would have to do with Steam, unless your gpu is tied to the Steam DRM, but while changing brands of gpu is not supposed to cause problems like it used to. There was a time when it was recommended to reformat Windows when changing from one mfg to another, so did you do a really, really thorough job of uninstalling the ATI/AMD drivers ?
Did you get your drivers from the Nvidia web site and after uninstalling the ATI drivers and associated components via Programs and Features ? Did you reboot and run Driver Sweeper to clean any leftover ATI graphics driver files,
and then install the Nvidia drivers after rebooting again when Driver Sweeper was finished ?
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