I never like the term "bottleneck" which implies that a stronger graphics card can somehow make your gaming go slower. A better card will ALWAYS get you better fps, but you will get diminishing returns if your cpu is weak.
In this case, your 750w psu can support a graphics card as good as a GTX590, not that I would recommend that.
Do not worry about the pcie-X16 speed. Only the very strongest cards are slowed down by pci-e speeds, and not in a meaningful way.
I think your plan for GTX560ti/GTX570 class cards is ok and appropriate for 1080P gaming.
I've been always saying in this forum that achieving higher FPS with a particular CPU is different than a real "Bottleneck". As long as a CPU is handling the game, no worries at all. And we've seen that till the current moment some high end games are more than playable on 2 cores.
With the exception for some games like Skyrim, such games prefer a faster CPU but it's still playable @ any speed with decent FPS @ ultra settings.