1) Is your motherboard sli capable? If not, the question is moot.
2) I always prefer a single gpu or single card where it will do the job. Some games may not scale well.
3) Your GTX260 is worth $50 to $70 on e-bay.
4) If you upgrade, you are looking at a GTX 579 class card, or you will be disappointed by the small increase.
5) Since your card sells for so little, you should be able to buy another one used on ebay for not much either.
If it works out, fine. If it does not, then you should be able to sell both cards without much of a loss, and get a single great card.
In this case, I say try sli. If you do, let us know how it works out.