Did you reinstall Mint after changing video card? That would probably be a good thing to do if you haven´t. Also the guide that you have followed seems make it a lot more difficult then it need to be. Every time I reinstall I just go to "System" > "Driver manager" and there I choose one of the proprietary drivers, (319 at the moment) which I´ve never had any problems with so far.
Desktop: Intel i5 4670K, 16 GB, Geforce GTX 770, LM 17 Cinnamon
"Steambox": AMD FX-6100, 8 GB, Geforce GTX 650Ti, LM 13 Mate
Ultrabook: Acer Timeline U, Intel i3, 4 GB, Intel HD 3000, LM 17 Cinnamon
Mediacenter: Intel NUC, LM 17 XFCE