Yes, this. I think you'd be better served by moving from the 260's to newer tech instead of adding another 260.
I'm running GTX 280's in tri-sli and definitely see the difference vs. the dual GTX 280 SLI set up I had previously (found a third one cheap when they were being discontinued in favor of the GTX 285). I should also note that I am running at 2560x1600 where the tri-sli really pays off. If you run a lower resolution, it is likely overkill.
For the incremental cost of adding another GTX 260 (~$200) you would likely see performance similar to a dual 470 sli setup, minus the DX 11 support (which is not yet widespread anyway) and save $500 in the process.
I'm planning to keep my 280 tri-sli setup until the next round of GPU's - perhaps GTX 485(?) if they bump the performance and deal with the heat/power issues. There is not enough performance difference to justify upgrading (unless I went to tri-sli 400 series which would be insanely expensive).