Generic recommendations:
64 bit processor - dual core
8 gig memory
1 Tbyte hard drive
If used for high end gaming, then high end video card
Don’t think about anything below a Quad core.
I have a mid-range for gaming (GTX 960) love it.
I used to add powerful gaming cards to a standard PC, but found it would run the PC fan too hot and burn out the video card.
The hight end video cards should be powered by a high end computer power supply (around 600 watts or so). They should also have better cooling systems, rather than a standard noisy fan.
I am convinced the best option to get a computer that can game a bit at a reasonable price is to go the lower end of a brand designed specifically for gaming. They will have the power supply, video card, and cooling system appropriately matched. I don't trust the big name computer companies to put together a good gaming PC, since its not the focus of their business (and they can remain in business even if they screw it up). Rather I prefer a brand that only does gaming computers.