Oh, it'll be very major, and trust me, the bottlenecking will be PAINFULLY noticeable depending on the game you play. Certain games are much better optimized than others, such as Battlefield 4. Battlefield 4 is so well optimized that you won't notice much, if any bottlenecking. However, other games, like Metro: Last Light, are not so well optimized. Metro: Last Light is often cited as a heavily GPU dependent game -- and it is. However, its CPU demands are often overlooked. It can be quite demanding on the CPU at times, and often you'll be hitting 30 fps, or sometimes even a bit less with an FX-6300, when you should be hitting closer to 50 or 60. How do I know this? I have an FX-6300 and a GTX 770, and I'm trying to replace my FX-6300 as soon as I can afford.
However, let me give some advice to your cousin. There is NO reason to get a GTX 780 Ti with a 1440x900 monitor. That's absolutely ludicrous. Few games can't reach 60 fps with the absolute max settings on a GTX 780 Ti at 1920x1080, let alone 1440x900. If he has a very underpowered GPU right now, I'd recommend a GTX 760 for 1440x900. However, you'll likely still be experiencing a lot of bottlenecking in some games. Because of that, I recommend your cousin get a new motherboard and an i5-4670k. He'll be much happier. Also, overclocking will do very little. The i5-4670k is about 70% faster in single-threaded apps than an FX-6300, both at stock speeds. For single-threaded performance (which games still can make heavy use out of) to match an i5-4670k an FX-6300 would have to be overclocked to roughly 6 GHz, and trust me, that's not going to happen unless your cousin has a seriously expensive motherboard and some fantastic cooling. Going the route of a new motherboard, an i5-4670k, and a GTX 760 will run your cousin around $600, and you'll still be better off than just shelling out more money for a GTX 780 Ti which would provide you with far poorer performance on its own by comparison.
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