How to choose a Processor

Intel or AMD?
This question is vital when you're upgrading, because AMD's and Intel's CPUs won't work in the other standard's motherboards, but it's relatively inconsequential when you're building a system for the first time. Though certain AMD and Intel CPUs do certain things better than others, those differences seldom matter much in terms of ordinary, everyday computing. So don't worry that you'll be cutting yourself off from certain tasks or aspirations if you choose one over the other. But one aspect of this choice is related to the previous issue: Every CPU in AMD's consumer catalog is available for under $225, while the most expensive Intel chip runs $999. Does Intel deliver oomph to match the money? In most cases, yes—but you won't necessarily be able to take advantage of it in every situation.

Number of Cores
It wasn't that long ago that the number of cores was an unheard-of attribute about processors—now, it's the one you'll want to focus on most. For all intents and purposes, every CPU these days feature from two to six processing cores, which all work together to crunch data and thus save you time. Not every software program supports this capability, and not all the ones that do support it equally. But software for particularly intensive tasks like photo and video editing will really benefit from a CPU with more cores. Of course, the more cores a processor has within the AMD and Intel families, the more it's going to cost (AMD's most expensive six-core CPU costs less hundreds than Intel's cheapest, for example), but if you're into heavy-duty computing you'll probably find the investment worthwhile. It's not impossible to find CPUs out there that have only one processing core, but on the off chance you do we'd recommend avoiding them. Sure, you'll save a ton of money. But even dual-core CPUs are so ubiquitous and inexpensive now that it's smarter to go with two or more cores than it is to select just a single-core chip.

In general, a multiple core processor is a better choice but that does not mean it is better for all conditions.
differences between the dual and quad core products, for example, Intel may have the quad core processor with a base clock speed be 3.0GHz but when only a single processor core is in use at full load, that processor core will be boosted up to 3.4GHz. This would then make the quad core processor just three percent slower than a dual core processor that runs at 3.5GHz.  Thus, if you have a program that is only single threaded, the dual core processor is actually better. Then again, if you have something that can use all four processors such as video transcoding, then the quad core processor will actually be about seventy percent faster than that dual core processor.

Source :
http://compreviews.about.com/od/cpus/a/dualcore.htm
http://www.pcmag.com/article2/0%2c2817%2c2374351%2c00.asp