THG did an article on 8GB DDR3. They recommend Kingston HyperX T1 DDR3-1600. Or Corsair Vengance DDR3-1600.
Check the THG article here.
Don't bother paying extra for speeds higher than 1600. Research has show that 1600 is the "sweet spot" in RAM timing for Sandy Bidge CPU (i5 & i7). Sorry, I don't have a link for the article.
And 8GB should be plenty, unless you do video editing, or something like that.