DX11 will not really bring anything new to the table. Not many games use Dx10. That said, it should be ovious not many will use DX11.
Buy the 260's.
i was the same as you i got told some good info ill share it with you if your gonna w8t untill the next big thing your wasting time because there will always be a next big thing grab them 260's your be playing so much you wont know the dx11 have even been realeased and they will probaly realese gtx 2** drivers for dx11 anyways that makes them more appealing to customers nerver regreted buying my system otherwise i wouldnt be buying my 3rd gtx 260 next month
by the time a game is made that supports dx11 youll be mad you didnt buy the cards now. i just upgraded everything myself, ive built my own rigs for the last 5 years. if you wait for the next best thing, by the time its here youll find yourself asking the very same question, should i wait for the next best thing?
i have asked that before and found my rigs not able to play the games of today, trying to wait for better technology tommorow, just get what you have found and have fun playing....
What is the monitor resolution that your using. If your at 1680 * 1050 or less than SLI isn't really going to do much for you. If your at a higher resolution and the game supports it, than it might be worth it. Personally I'd just stick with a single GPU solution, since your going to save energy and not require a more expensive PSU to handle the GPU's.
SLi or crossfire in my opinion is something you do to upgrade; you buy a really fast GPU then a little while later its not so fast so you add another GPU (cheaper this time) and increase your speed.
Unless your an enthusiast and want the most possible performance and then your not buying a gtx 260
As for waiting for the dx11 cards, I don't see the major plus as dx11 but as better, more powerful, GPU's, will dx11 compatability as a bonus (which I know will take a while to become standard but when it does your already ahead of the pack, now with dual 5870's)