Honestly, the GTX260+ and the 4870 1GB are for the most part the same in terms of price/performance. The GTX260+ edges ahead ever so slightly in a few places, plus gives you PhysX if you're into that sort of thing. The 4870 on the other hand lacks PhysX, but in a few places blows right past the GTX260 and even the GTX280 (take "blows past" with a grain of salt, in current reviews the notion of "blows past" seems to mean a handful of FPS most of the time).
My suggestion - go with whatever your gut says. You won't be disappointed performance-wise either way.