All the benchmarks say go with the 4870 1GB for the most part. I just made this decision myself (it was very difficult). I decided to go with NVidia with the GTX 260 216.
One thing I noticed in benchmarks with the NVidia card is that a lot of them did not score as well on average framerates as the ATi solutions, but for reviews that had min framerate also posted the NVidia card were almost always higher. Sometimes by a good margin. I would rather have high minimum framrates, then the absolutely highest average.
Also I much prefer NVidia manufactures, they usually have better warranties, tech support and with EVGA and BFG they have a awesome trade up program.
And this is coming from who's last video cards were a 9800 Pro, X850XTX PE, X1950XTX PE and I had a HD4870 1GB, but it had a faulty fan and I had to RMA it. I got a BFG GTX 260 MaxCore OCX instead. As a matter of fact I just got it and installed it 1 hour ago and am currently stress testing it.
Also for OCing, the 4870 runs decently hot so its a little hard to OC and keep the fan at a tolerable level. And for the 1GB version it has higher density chips that do not overclock nearly as well as the 512 version. if you were to the the 4870 though definitely go with the 1GB version, its a good performance increase and OCing the memory on that card does not give much for results.