The vast majority of games only use 2 cores. However, if you know your are going to buy games that can make use of 4 cores, then go ahead and just get a quad core CPU. Just be aware that the number of games capable of making use of more than 2 cores is very small.
As for using two monitors... if you were only surf or have itunes up and running then you can rely on a single graphics card. However, if you are going to watch movies (especially HD Videos), then the best options is to buy a 2nd low power graphics card as mentioned by popatim.
Playing a game and watching video can cause a 15% - 20% decrease in performance in the game and may result in choppy video from time to time. That's because the GPU is trying to render the game and decode the video at the same time. Opting for a 2nd inexpensive graphic cards like a Radeon HD 6570 or a nVidia GT 620 is going to be better than spending an extra $100 - $200 for a more powerful single graphics card because you are likely still going to get stuttering when watching movies.