As already mentioned, the i7 875K will also be better if you plan on performing video encoding / transcoding operations. The 875K has Hyper-Threading, allowing your CPU to process 2 threads per core, per cycle. It's almost like having an 8 Core CPU, and Windows will treat it as such.
For direct comparisons though, the i7 875K is more similar to the i7 960 from a performance point of view. Only major difference being the wider memory path on the 960 (Triple Channel).
I would not worry about over-clocking killing off your CPU early neither. As long as you can keep the temperatures within safe limits, the CPU will live on for just as long. The 875K will be better for this because of the unlocked multiplier, and it will save your system RAM taking a pasting. Usually over-clocking the CPU will cause the RAM to over-clock too. Tweaking the multiplier and clock ratio of the CPU can prevent this from happening, and is more desirable (unless you have some RAM you can push to 2.2 Ghz+ or something. The 875K is a fantastic over-clocker - You can get them to 4 Ghz pretty easily.)
Intel do have some new CPU goodness due out in weeks, and it's widely anticipated to be very good. You might want to wait.
As for 6 Cores - You are right about them being of limited use in gaming, but they ex-cell really well in wildly multi-threaded programs . Intel's processors on a clock-per-clock basis are better than AMD's, but it does not mean AMD's 6-Core CPU's are bad at all. Many people are attracted to AMD's CPU's because of the lower platform costs overall. You seen how cheap an AM3 motherboard is compared to an i5/7 compatible one?
AMD's 6 Core Phenom is about as fast as Intel's quad core i5 line-up in the real world. I would lean towards a 6-Core Phenom 1055T for the same money as an i5 760 personally, but each to their own.
Mini Beast - Intel Core 2 Extreme QX9770 [4.2 Ghz] | Gigabyte X48T-DQ6 | 8GB DDR3 | KFA2 GTX 750 Ti | Seagate 2TB SSHD
Mega Beast - AMD FX-8350 [4.7 Ghz] | Gigabyte 990FXA-UD5 | 16GB DDR3 | Tri-Fire XFX DD R9 290X | Sandisk X300S 512GB