Well, Witcher 2 is not a game I'm that familiar with. As far as the consistent performance, eh, heres one article showing some of the differences, yes, its using an 8150 not the 6100, but incidentally, it also shows you how these games are doing with an 8 core CPU up against a quad (Phenom II), for good measure they threw in the 1100T Phenom II which is a 6 core.
http://www.anandtech.com/show/4955/the-bulldozer-review-amd-fx8150-tested/8
As far as personal experience, for the record, I don't own a 6100 myself, so I couldn't tell you what my personal results are for it. But yea, I have a quite a few games I've played. Metro 2033, BF3, BFPlay4Free, all of the COD games (including Black Ops2). All of them, by my standards run flawlessly @ 1080p.
My ex had an i5-2300 w/ a GTX 460, and TO BE FAIR, at the time I had an overclocked 550 TI, both video cards perform about the same, and neither were up to snuff to max out more modern games at such a high resolution like my 7870 is. Also, the i5-2300 is not really a big reflection on better i5s and even some i3s because they had a terrible clock speed (2.8GHZ)
We played in 1280x1024 at the time... There wasn't any discernible difference in gaming performance of either rigs. I mention it simply because, like you said the budget is tight. If you're looking at a entry-mid level graphics card, you won't see much of a difference buying heavier on the CPU anyway, since you're going to run into a video card limitation.