What difference in performance in games like oblivion in fps does it make between a core 2 duo e6400 standard at 2.13ghz and it overclocked to 2.77ghz assuming same componets? And if it makes any noticeable difference, does it justify the 100$ difference between the 965 and 975 chipsets because most 965s cant be overclocked?(ignoring i680 nvidia)?
To answer your question, a couple fps depending on what kind of graphics card you have.
But what do you mean most P965 boards cant be oc'ed? The P965 chipset is great for oc'ing! The Asus P5B Deluxe, Asus P5B-E, Gigabyte DS3 & S3, MSI P965 Platinum, Biostar TForce965PT, just to name a few, are all awesome for oc'ing. The P965 chipset is just as good or even better then the 975X chipset when it comes to oc'ing.
And the 680i chipset and P965 are to completely different things, the P965 is an Intel chipset, the 680i chipet is made by Nvidia.
Not sure what you mean by "whmkr" but the G965 chipset is used mainly in micro-atx mobo's, not something an oc'er would likely get. Im assuming that your friend got a P5W, which is a great oc'ing board (Although i would have gone with the P5B Deluxe). Its not really the chipset that makes its good, its the BIO's features and overall quality of the board.