TurtleBarbecue: For Witcher 3 I'd r9 270 is absolutely minimum. There's no way you will be able to play on r7 240. Don't look at VRAM ammount. And with this card you'll still be able to run 720p minimum/low only. If you're going to build new PC I'd say GTX 970 or at least 960 is reasonable choice. Forget about cards you mentioned.
Are you sure? R7 are pretty old nowdays. Maybe you are right. Can you give me an equivalente to R7 240 from previous generations?
To OP. I am not sure, but I think you will be able to run the game on the bare minimum with 25 fps with 8570.