The i5 760 runs at 2.8 ghz, has 4 cores, and turbo; it sells for about 210 in the US
The i5 660 runs at 3.33 ghz, has 2 cores, hyperthreading and turbo; it sells for about the same price.
The 660 has graphics built into the CPU. The 760 does not have this capability--you would have to buy a separate video card or get a motherboard with onboard video.
In turbo mode, the 660 runs at 3.60 ghz
In turbo mode, I think the 760 runs at 3.46 ghz
Turbo mode forces the processor to operate at a faster clock speed, like overclocking.
See here:
http://www.youtube.com/watch?v=llOXMPXH2VA
and here:
http://www.intel.com/technology/product/demos/turboboost/demo.htm?iid=tech_tb+demo
The 660 uses less power and generates a little less heat.
Both are socket 1156.
Both are excellent choices.
The 660 has a noticeably higher clock speed and would likely be the better choice in single threaded applications.
The 3rd and 4th core of the 760 may be a deciding factor, depending on exactly what applications you use on your PC. Or those extra cores may do you no good.
For the vast majority of cases, you won't be able to tell the difference without a benchmark.
Here is an explanation of hyperthreading, which is found only on the 660.
http://en.wikipedia.org/wiki/Hyperthreading
It is a way to get more workload on each core, as a kind of simulation of another core.