Nvidia has really good support for older models with gains from Drivers for years to come. But as for SLI'ing, they won't seel them forever, how long were you looking to wait to buy the second card? It usually happens that if u wait too long it's more worth the money just to buy a newer card.
You should be able to get a new GTX 560 Ti for a long time to come... I would assume the remainder of this year and well into next year. The 600 series replacement is still not even scheduled for release at this point. If you upgrade to a second card, you should have no problem finding one at any point over the next 1-2 years. Checking on Newegg shows there are still plenty of the previous generation GTX 460's available, which were replaced by the GTX 560's over a year ago.
thanks for the replies, are still considering, as i'm not swimming in money at the moment, and might buy a laptop instead, as I need that as well, but thanks for the feedback, ang good to know that I can buy one now and get another later.