To be honest I would skip the whole SLI thing and buy a 7850. For running lots of mods you need lots of vram. SLI won't help you at all in this aspect. 1gb isn't gonna cut it. Spend an extra $20-$30 and buy this card( http://www.newegg.com/Product/Product.aspx?Item=N82E16814161405
). It is super cool and a overclocking beast. It is much better than a single 560ti and that extra vram will help you out alot.
Hope I helped!