OK, top 3 most important features !!
1. GPU - the graphics processing is the most important thing, look for Nvidia GTX 700 and 800 series.
2. The CPU - buy the best you can, obviously Intel i7 quad core is best, but an i5 would be OK too. Dont get the AMD CPUs in my opinion.
3 RAM and VRAM - 8Gb Ram and min 4Gb Vram
Obviously, play before you buy and check out the screen resolution !