Also, if you don't need to upgrade, why would you?
I say keep the 2x GTX 260s in SLI until they are not powerful enough to satisfy you anymore.
One card will not be as hot as two, will be quieter (especially if an after-market cooler is added), and will use less power.
I have a review of a GPU cooler that is being proofread right now, but once it is published would be well worth checking out for this.