If your applications are multi core capable, then the i7-860 will be the winner.
If your tasks tend to be single or dual core intensive, then the i5-670 would be better.
But... consider the i5-660, it is about $90 cheaper, and only one multiplier slower. I have been testing one for a friend who does scientific calculations, and it is a screamer. It does 2m superpi almost as fast as my overclocked i7-920.
As the 860 has a very good turbo mode, with 2 cores active the frequency jumps to 3.33GHz according to reviews I've seen, so even if you only used apps optimised for dualcore *at the moment*, you'd still get great performance - so for roughly the same price it's a no-brainer to get the 860 as you get a full quad core chip.