I don't know much about the GTS 250s or the 260 but here is a general guideline on SLI based on my experiances with SLI.
1. A SLI setup with 2 or more cards will out perform a single card of the same size. Obvious example; 2 GTS 250 are faster than one GTS 250.
a. How much more power? 1.7 maybe? (That is probably a very generous guess)
2. With SLI you really need to look over your whole system carefully. 2 cards require more power, more space, more cooling, more wiring, more money etc.
3. I would suggest using the best single video card you can afford unless you are building a top of the line system. Then think again if you really need 2 video cards.