nekulturny :
With the graphical limitations of a 6850, yes the FX-6300 will be sufficient. However, as to giving you a figure of how many years it will last, it really comes down to your performance expectations on that one, and what games you're getting into. If you're talking Battlefield 3 @ 1080p fully maxed AA and all that jazz, well your 6850 won't do that for you today, even with a CPU upgrade. But, COD maxed out @ 1080p isn't too much to ask for.
Hey mate, no offence intended but I really think you don't quite know what you`re talking about when it comes to 6850. On my current system, which by your standards, is a "dinosaur" : Gigabyte H55M-UD2H, Core i3 530 + Titan Fenrir cooler (OC @4.2 Ghz stable 24/7), a Sapphire Vapor-X 5850 (OC @ 850 core / 1200 mem) and 8 Gb Corsair XMS3 @2000mhz CL9, I am able to max out (8x msaa and 16x anisotropic) ANY game that is out on the market now.
Battlefield 3 runs at ~40 fps, Assassins Creed3 60-80 fps, Far Cry 50+ fps.
This was by no means a mocking post, just a reminder that you are way off track with your estimation of the earlier generations of video cards` performance.
Bear in mind that the 7850 is only about 10-15% faster than the 6850, which in turn is marginally better than is precursor, the 5850. Also, the 5970 mops the floor with the brand new 7870.
So overall, the performance gain from one generation to another of GPU`s isn`t anywhere near as big as you imagine it.
L.E. Seeing the current products available, I would say that, in almost every system configured by users, the CPU acts as a bottleneck for the GPU. The current GPUs are way too powerful for the CPUs available. For instance, there isn`t a single cpu out there that won`t bottleneck a gtx 690. Not even the i7-3770k can keep up with that gpu.
All in all, my advice for the OP is go for the i5, as your gpu can hold any current games or any games that will come out in the next year or 2. (maybe in like 2-3 years you will have to lower your details a bit but that is a minor inconvenience )