The only people who will buy the GTX 260 or 280 (once the 4870x2 comes out) are idiots who don't research anything.
There is absolutely no reason to get the GTX 260 since the 4870 is more powerful at $100 less. Also, once the 4870x2 comes out it will destroy the GTX 280 in performance and be $200 cheaper (4870x2 has a projected MSRP of only $450 compared to GTX 280's ridiculous $650+ price tag).
The ONLY card that Nvidia will have to compete with ATI in this generation is the dual-GPU version of the GTX 280 they're going to release (who knows when), and that will more than likely have an MSRP of at least $800, effectively making it out of reach for the bulk of people in the market for a GPU.