Building a new Computer

Hey everyone, I have been out of the loop for a while now when it comes to building computers. I have been asked to build a gaming computer and I can easily build them I have built loads. However I don't know what the new things are and whats the best for certain situations.

Basically he wants the computer for a lot of simulation games like Flight and Train simulator. He likes to have them on high settings and wants good frame rates.

Any advice on anything would be great.
Thanks, Jack