1. A single card setup always comes with less hassle than dual card setup. A kids knows that. I can fill up this post with 20 Points to do that.
2. 660 what ? a good card, but an old one. Adding an old card over another to achieve a mark of performance which already can be achieved with single card setup which comes with latest GCN, DX12 support, Mantle support, Game bundles? Stupid idea.
3. 290 comes with more effective VRAM, more memory bandwidth.
4. 290 performs as good as a 780, where 660 (non ti ) SLI will struggle a bit to reach it. Raw performance.
5. OP in his life if wishes to dump that card for some money and go for a new card EVER!, congratulations you have two even older cards to dump instead of one.
6. Need of beefier PSU, more noise, more elec bill in any dual card setup, even if all these points go null and void in front of a 660 SLI and 290, even then what purpose does it serve when op can sell his old card, add the money to grab a new architecture 1440p ready single card setup, on which this forum has been so optimistic ever since its launch, despite its high load temps and noise issues.
I expected a more sagacious post form you for you have been here for a while but well...
Feel free to to ask me to know more about single card advantages over mult-icard anytime.