Generally speaking the more powerful the graphics card the larger the bottleneck will be for a Core 2 Duo CPU. However, the faster the C2D CPU, the lower the performance bottleneck.
The NVidia GTX 550 Ti is not a particularly powerful graphics card and you should not really have any bottlenecking issues with it. It's performance is between the Radeon HD 5770 and the Radeon HD 5830 back when it was released. Translated into more modern cards, it's performance would be between the Radeon HD 7750 and the HD 7770. The GTX 650 (vanilla version, not the Ti or TI Boost) is slightly more powerful than the GTX 550 Ti.
I currently have a C2Q Q9450 and a Radeon HD 5850 graphics card and I am not experiencing any bottleneck. Yes, it is a quad core CPU, but many games still only use 2 cores. For example, if I stuck the Radeon HD 5850 with a C2D CPU clocked at 3.0GHz, I should basically have the same performance when playing Skyrim since that game only uses 2 cores.