Keep in mind that the worst-case scenario w/regards to a CPU bottleneck when upgrading from one single card to another single card w/the same proc is that you'll get the same FPS that you do now ... only you'll be able to achieve this same FPS w/much higher graphical settings. And the worst-case scenario is going to be rare.
Also, whether or not X CPU will bottleneck Y GPU depends on the particular test (i.e. what game/bench) you choose to run, and what settings you choose to run that test at (what res, AA level, IQ settings, etc) in order to try to detect the existence of a bottleneck.
BN's, in other words, are situational, and one cannot accurately answer this question unless you also include the above-mentioned information. On 3dMark06 using it's "stock" settings, the correct answer would be "somewhere around a 7900GTX". Whereas on Metro2033, totally maxed out in DX11 at 2560x1600, the answer would be "somewhere around Tri-SLI GTX580s".