You should take a look at this article:http://www.tomshardware.com/reviews/build-balanced-platform,2469.html
Your cpu is comparable to the E6300 that they use, and the GTS 250 is comparable to the Radeon HD4850.
The answer is that it will depend on the game, and the resolution you use. Some games are more CPU intensive, and some are more GPU intensive. If you run games at higher resolutions (1920x1080 and up), your performance probably won't be limited by your processor. At lower resolutions it will depend upon the game, but in most cases your system shouldn't be CPU limited.
It obviously wouldn't hurt to overclock your processor. This follow up article covered the same processors while overclocked:http://www.tomshardware.com/reviews/balanced-gaming-pc-overclock,2625.html
While overclocked the E6300 was the only processor that caused any bottlenecking and only in GTA4 and only at resolutions of 1680x1050 and 1280x1024.