Nope! No bottleneck in your system, even at higher resolutions -- the display size does not matter -- I have a 42" monitor and it makes no difference when I use my 23" one.
I'm confused by what you're saying. Higher resolutions and display size are two separate things. If your 42" TV is the same resolution as the 23" one, than yeah, there is no difference as far as the GPU is concerned. If you're talking about playing on a 1080p monitor versus a 2560x1920, that's a difference of some 3,000,000 pixels being rendered, and a single 680 will not be enough to run that resolution at the highest settings in certain games. So yeah, in that situation the GPU would eventually become the bottleneck.