Quad core, quality i5 or better yet, i7, 960 graphics card and good memory should do it. A SSD is a must if you want fast load times.
The hyperthreading of an i7 is, to my understanding, not really used in games. But quad cores are. And the i7 has other advantages with certain games. As some games are very CPU intensive.
You may read that the i5 is only 3 frames per second slower than the i7 in benchmark tests. However, in some games that's not true at all. It can be as much as 15-20 frames per second with certain games and simulators. As the CPU is bound up with other tasks.
Another interesting option is to look on ebay for a used HP Z400. Sometimes these can be purchased with just the right equipment.