I don't see anything wrong with this thread, so I'll reply with this helpful chart:
http://www.techspot.com/review/405-the-witcher-2-performance/page4.html
Witcher 2 is an older game, so you won't see new cards benched with it. But the 7870 is between a GTX 570 and 580 in performance, so as you can see it will probably be just fine for you. It's discontinued, however, so finding one in stock may be a challenge. Newegg happens to have a Powercolor model on sale right now, but it's likely in limited quantities.
The GTX 760 would definitely be more powerful than necessary for that game, and frankly too large an investment for your resolution.
The R9 270X, at its inflated price, isn't a worthwhile purchase right now. At most I'd pay $200 for it, but a $150-160 HD7870 is a better choice if you catch one in stock.