I play the witcher 3 on:
Athlon 64 X2 3800+ (Windsor, Socket AM2) with light overclock to 2200 Mhz, runs stable at 50-55°C
Gigabyte Geforce GT 630 2048 DDR3 (not Overclocked)
4GB Ram
I tweaked the Witchers User.ini to lower tesselation, Speedtree, and Decals (Decals are additional cosmetic environment objects like little stones on the ground). I also lowered the resolution to 800x600.
Does look good enough and runs like a charm. No stuttering sounds and fluid gameplay.
Thats maybe not even the lowest spec it runs on without lag.