winrar uses 4 cores but is not efficient at it If I remember correctly.
Most mainstream programs don't need to use 4 cores so they don't.
programs such as video editing and rendering programs will use as many cores as there is. 7zip uses 4 cores.
So newer games will use 4 cores. Games made on the source engine uses 4 cores. Battlefield 3 multiplayer uses 4 cores. Its about 50/50 whether a game uses more than 2 cores. Generally speaking tho 80% of game will run better on the i3 even if they use more cores because they really only use 2 main threads and the i3 cores are better.