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Official BM Config Gude - http://www.blackmagic-design.com/media/2554788/DaVinci_Resolve_Windows_Config_Guide.pdf
To be short - good, latest multicore CPU, big amounts of RAM (16GB or more preferable), and multiple top NVidea graphics cards.
For Black Magic having compatible GPU is almost requirement. It works only with NVidia cards, btw. And top ones, with many cores.
Other important thing is that it like cards with big amount of RAM.
The more GPU cards you have - the better. Free Lite version can't use more than one GPU, btw.