Bit of confusion, I suspect.
Firstly, the i5-3570K is a bottleneck, for nothing. It may become so, if and when new graphics cards arrive, which are much more powerful, than anything now available. Nobody knows when that may happen, so I shouldn't worry about it. The i5-3570K is effectively the most powerful, today, and for some time to come. The Haswell series, due out next year, may have a more powerul one, but no-one knows for sure, what and how much.
The GTX660 and GTX660Ti are two differently powered cards, the Ti being about 20% better, but around £60 more. Possibly one of the best of either, is the twin-fanned versions, by MSI or Asus. The 660, in terms of overall performance lies between the Radeon HD7850 and HD7870, but beats the HD7870, in some games, like BF3. The 660Ti lies between the HD7870, and the HD7950, and beats the HD7950, in simliar games.
Any of those, will be "adequate", you just get more, if you pay more, and whether Radeon, or nVidia, are better, depends on what games, you particularly want to be good at.