The Core i3 is all you need for general "stuff". Also most games still only use one or two cores. Casual games will definitely fall into that category. Even a few "AAA" games only use one CPU core like the Assassin's Creed series. The integrated Intel HD 4600 is also fine for casual games, and you can always install a graphics card for better performance if necessary.
The Core i5 is a great CPU. But if you know you are doing fairly basic things like surfing the internet, watch movies, use MS Office / Open Office, then the Core i5 is more or less overkill.