There is no "Sandybridge" Q6600...you mean "Kentsfield" right😕My CPU is a Sandybridge Q6600 not the newer Ivybridge.
No - PCIe 2.0 is not going to bottleneck you on Single GPU setups.
You will have a bottleneck with your Q6600 though. (and it's not Sandy Bridge, its a Core 2 Quad Kentsfield)
If you O/C that Q6600 to 3.5 GHz or so, it will allow you to play pretty much any game but will still bottleneck that GPU from everything it has to give.
If your running your Q6600 stock, your really bottlenecking that GPU.
Your ready for a CPU/Mobo/Ram upgrade.