So you haven't built yet, and you were considering an HD 7870, but now you think it's better to SLI two low-end cards? NO.
Sound advice: if you want to go Nvidia, buy a GTX 660. If you don't mind AMD, buy an HD 7870. They're about equal in performance with the 7870 winning more often, especially when both cards are overclocked.
Either way, a powerful single GPU is always better than any mid-low end dual-card config.