Depends on you. After all there are a few situations where SLi does not scale well. On the whole though two 260s are faster than a single 280. Still if you get a 280 now you can save up to buy another one latter so that you can SLi two 280s.
yeah and considering that the Superclocked GTX 260 Core 216 is 1~3 fps slower than the GTX 280. most of the time they equal the GTX 280. so if you SLI GTX 260 Core 216 you will spank anything out there (except SLI GTX 280's but youll beat 4870X2 or CF 4870's). also the Core 216 is faster then the 4870 in most cases so think about it. if you decide i recommend the EVGA GTX 260 Core 216 regular and overclock to the Superclocked editions speeds. you save a little cash that way.