It would depend on how much you willing to spend really! You can get a budget graphics card etc and the games will work fine, however if you want to play the games on high detail then you will need quite a powerful pc.
I would recommend a good prcessor (around 2.0Ghz) and a good Graphics (these days 9800pro) card and lots of RAM. These specs. might look a little high but personally speaking I hate if there is any kind of lag problem with my FPS.
Are there any models from manufacturers that u would recommend because I don't have time to build one