¿ i5 4670k, i5 3670k, fx 8350? Which of them is the best

4670k would be best in most of the games. They're about even in BF4, the FX may pull ahead in Crysis 3 depending on the settings. If they were both at 4.4 the 4670k would be ahead.

I'm curious why you want to run 760s in SLI - you could probably get a better single card solution for the same price. Dual graphics solutions still aren't supported by all games.

If it's just for gaming go with the i5. If you're looking to do other productivity things such as video editing/3d modelling the FX is a contender due to its higher core count.

Here's my draft for the difference between them:

The difference between AMD and intel for gaming.
Firstly, you need to decide what your priorities are, and what you will use the PC for.
Things such as: light gaming, heavy gaming, basic work (e.g. MS Office), heavy work (e.g. video editing, 3d modeling).
For the most part in current games the biggest difference will be made by the selection of the GPU. Get a great GPU + worse CPU rather than worse GPU + great CPU.

The AMD FX CPU's have many cores, which are weaker.
intel i5's have less cores, which are stronger.

The intel's consequently have better performance per core. In older games, the intels perform better as those games are optimised for good performance with only a few cores (single-threading).
In newer games, the AMD FX's really shine due to the introduction of games using more cores (multi-threading), which may continue into nextgen (unconfirmed).

The difference comes in depending on what you want to use the PC for. If you're on a tight budget, save some money and go with the AMD and spend the extra money on a better GPU that will give you better performance than any CPU could.

i5: Good for older games (single-threaded), Good for newer games (multi-threaded), Good for general work, great all-round CPU and probably the best around for current games (may change in future).
AMD: Slightly worse for older games (single-threaded), Great for newer games (multi-threaded e.g. BF4, Crysis 3), Good for light/heavy work, extra cores are great for 3d modeling and video editing or rendering, great CPU whilst costing much less than the intel. Even though it's worse in older games it will run them perfectly well and smoothly.

Regardless, both will perform well.
For an i5, I would recommend an i5 3570k or a 4670k. Why? They are king for gaming performance at the moment and since they are the k version they are unlocked and can be overclocked in future for a performance boost.

For an AMD, I would recommend a FX 6300/8320/8350 (might as well get the 8320, it's an 8350 clocked lower at stock which you can change) [Do NOT go with a bulldozer CPU, only piledriver. List here: http://en.wikipedia.org/wiki/Piledriver_(microarchitecture) <-- That should all be one link, not sure why it splits.]. Why? Great multi-threaded performance for newer games and heavy work, are just fine in older games (not overkill, can deliver smooth frame rates maxed with a good GPU), and are great for productivity with a tame pricetag.

As a general guide for gaming: (FX's piledriver, intel's sandy/ivy/haswell)
- FX 4300/4320/4350 = i3
- FX 6300/6350 = i3 or mid i5
- FX 8320/8350/9xxx = i5 (k) / i7 (well-threaded games, streaming [i7 hyperthreading isn't very beneficial to gaming]).


In conclusion, budget gaming/work: AMD. Not on a budget gaming/work: i5/i7. The i5 currently delivers better performance but don't get the impression that the AMD is lagging behind. They are great for gaming and work with a really great pricetag, just not currently up there with intel. In newer games though such as BF4 the AMD's have caught up in performance and in some cases deliver better performance than the intel's for much less money. You will get great, smooth FPS with either.
Either solution will game just fine with a nice GPU, focus mainly on that.

Some non-synthetic benchmarks between the FX 83xx series and the i5/i7's: http://www.youtube.com/watch?v=4et7kDGSRfc & http://www.youtube.com/watch?v=eu8Sekdb-IE The i5 will win in the majority of games, but if you're over 60 FPS (which you're very likely to be) you won't see any difference.

TL;DR - FX 6300/8320/8350 or intel i5/i7 k. Get the best GPU possible (save some $$$ from cheaper CPU), any will be fine.

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Also, I'd reccomend an FX 8320 over an 8350. It's essentially an underclocked FX 8350 at stock but it's much cheaper. The overclock difference between the two is negligible (around 50 MHz or 0.05GHz) so if you're planning to overclock anyway go with the FX 8320.