If it were me I'd just get an inexpensive non-sli board. I used to have SLI but it sucked so I sold my second card. Plus you won't have to buy a new PSU either. you'll be saving a lot of money so you can get an awesome card in the future
The only problem i'm seeing is that when games demand more power, I won't be able to cope up with just a single gtx260. So I would like to go with sli to more future-proof and not wasting my gtx260 by buying a different one in the future.
If you were to go SLI in the future I believe you would need to begin looking at more powerful PSUs. It's dependent on what a 260 requires (I have no clue atm), but if so, it would be an additional cost to consider.