this will ilicit flames from both sides of the fanboy territories...
tbh though, both cards have very similar performance. its best to look at the games you will be playing most. if you playu games that favour nvidia card, go that route. and vice versa.
personally, i would go for the 4870 since from my own research and experience its the more powerful card. but only by a small margin. and if you plan on playing the hell out of crysis or anything that uses physx, the the 260 is the way to go.