The GTX 260s may be sweetened by their OCs. If they can get to 700 like many GTX 280s seem to be doing. We don't know what the max OC for the 4870 will be and it may have a smaller range. Both [H]ard and Legitreviews only got +30MHz on the core with their HD 3870s. By contrast [H]ard got a +94MHz core bump on the 260 they tested. That gave them +7FPS @ avg 1920x 2xAA|16xAF in AoC. They only got their GTX 280 to 675 and that's still 73MHz over stock. We will see as user results start to filter in.
Also for both the GTX 280 and 260-
Looks like PhysX is going to have a larger impact in the near future-
"We also spoke with Brian Burke (Senior PR Manager) and Roy Taylor (Vice President of Developer Relations) on the phone and they had a ton of things to say about PhysX and were obviously all for it. They were happy to announce that 14 to 16 new PhysX game titles would be available before Christmas and double that next summer
Other nice things about getting a 260 (or any other nVidia card) would kick in if you buy one from XFX or Evga.
If you get it from XFX you may transfer your lifetime warranty to another person. If you resell your cards when you are bored with them or to feed another purchase this fact will build value to your buyer. Aka you get to ask for a higher price. Other cards have their warranties voided when they leave the orginal buyers hands.
Evga have Step-Up so if nVidia release some uber card within 90days of your purchase you may trade in your card and pay the difference.
Both companies officially support overclocking, so you don't void your warranty by doing it. Both companies offer lifetime warranties. Both companies support users replacing their HSF without voiding their warranty.