If you go to the websites of the various games you want to play, you will generally find a page with the minimum PC specifications for playing each game, and the recommended specification. That's a good starting point. You'll find some differences: my son plays Skyrim and Battlefield 3, and while he gets very good performance on Skyrim, performance on Battlefield 3 is less good with his set-up.
Then it's a question of what level of performance you will accept vs. how much money you have. The minimum specification recommended by the game manufacturer will give you adequate performance, but frame rates won't be high and you'll have to compromise on the level of detail. The recommended specification will give you excellent framerates and high levels of detail - but will generally be significantly more expensive (the difference, often, between a dual core processor and a quad core processor, and the difference between a £50 graphics card and a £250 graphics card.)