A half-way decent video card will be the most important I would think. If you run games at high resolutions. I managed to *build a decent computer that I am sure would run those games.
*I haven't built it yet but it is ordered.
Go on NewEgg and make yourself a wishlist of everything you need.
Here is mine. Everything but the videocard is fine. I am replacing the video card with a nice NVidia card.
Has everything except a monitor. I was very pleased with the price.
Hope this helps. Also If you use a setup similar to mine, spend the extra money on a good video card.