Both the cards can easily max Skyrim and Fallout 3. However, with the Nvidia Card you get Physx and some other Nvidia features in many games.Its about your choice.Personally, I would go with the MSI GTX 750 Ti.
Installing a graphic card is very easy you just have to remove the old card from your motherboard and uninstall its drivers and put the new card and install its drivers. (Don't use the driver cd that comes with your card in the box for installing the drivers. Go to the card's website to download the latest ones.)
If you are still confused about how to install a graphic card , you can check any video on Youtube.