It really depends on the game... According to all the review sites I've read on these two, the GTX260 c216 outperforms (albeit only slightly) the 4870 in the majority of newer games, especially with high AA and AF and higher resolutions...
As far as the GTX260 costing more, remember, you're paying for the same amount of speed as the 4870, the rest of your money is paying for better drivers... Nvidia has always had great driversets for the last few generations, while ATI has had horrible drivers for the last few.
All that being said, you can't go wrong with either card! Both are fast, but they're both fast in different ways... different strengths and weaknesses.