Try running a benchmark like Unigine Heaven 2,5 and/or Futuremark 3D11 or Futuremark Vantage and compare them yourself. Make sure you use the newest video card drivers. Those programs are free and will take into account your particular computer set-up better than anyone's opinion.
Both of those cards are DX10 BTW.
Thanks for the replies. I did notice that the 4870 is DX10.1 capable while the 260 is only DX10 Capable. I tested them both out and get fairly similar results in most games. I'm not totally sure if DX10.1 adds anything important over DX10 though. Any other inputs on these cards would be great.