All we can do is guess. It needs a reasonably modern GPU, because of the DirectX 11 requirement. Cards with 16 ROPs and 256-bit memory buses were the stated minimum for Witcher 2, and I think 16 ROPs and 128-bit (because the faster GDDR5 is common now) will be the playable minimum for Witcher 3.
Witcher 2 was visibly constrained by budgets for main RAM and VRAM, and I think Witcher 3 will want more. 8GB main RAM, 2GB VRAM.
Witcher 2 was not noticeably CPU-bound but did perform better with more than 2 cores. I think older 4 core models (Intel Q6xxx, Q9xxx, AMD Phenom II 4x) and newer 4 bogocore models (Core i3, AMD A8, AMD FX-4xxx) would be enough, if it weren't for memory bandwidth. Memory bandwidth may be much more important than CPU core count or instruction crunching, and it's the Ivy Bridge, Haswell, and AMD FX-8xxx CPUs and their chipsets that excel at this.