Once you know whether you're buying an AMD or Intel CPU, and how much money you'll have to spend, you need to think about the motherboard socket into which the processor will fit. These evolve over time as new technical developments and processes require new hardware, and the differences between them can be confusing. AMD currently has two main sockets: FM1 (for use with its APUs) and AM3+ (which, in addition to accommodating its current high-end Bulldozer-based CPUs, offer strong backward compatibility with several prior years' releases). On the other hand, Intel's mainstream processors now use the LGA1155 socket, whereas its enthusiast models use the LGA2011and you can still find chips for older Intel sockets like LGA1166, LGA1366, and more. As mentioned, lower-priced CPUs for one socket type are usually going to be slower than others of that type, so if you think you may want to upgrade again soon, take a look at what else is available so you make the right decision.
Number of Cores
It wasn't that long ago that the number of cores was an unheard-of attribute about processorsnow, 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.
Note: Related to the number of cores a processor has is the number of threads it can process. A thread is a string of instructions from one of the processing cores, and software programs that can manage more than one of these at a time will generally be faster than similar programs that can't. (If you see the word "multithreaded" in a software review, this means the app can handle more than one thread simultaneously.) All AMD-based CPUs turn out one thread per core; some Intel processors use a technology called Hyper-Threading that mimics multiple threads within cores, giving you essentially twice the threads for your money. For example, though a four-core AMD CPU may be limited to four threads in an application, certain Intel quad-core models may be able to juggle eight in the same applicationwith a healthy performance boost (if not necessarily twice the speed).
Back when all CPUs had just one core, clock speed (or operating frequency) was king, and the surest way to determine how fast a chip you were getting. Multiple processing cores are the most useful initial determiner now, but clock speed is still crucial. These days, it's measured in gigahertz (GHz), or the number of billions of times per minute the CPU's clock "pulses" into the microprocessor. A 3GHz CPU, then, pulses three billion times per second, and it's generally safe to assume it will be faster than a processor with a similar number of cores that's rated at only 2GHz (or two billion pulses per second). On multicore processors (and when using multithreaded software), the number of pulses stack, so you can expect a four-core 3GHz CPU to trounce a dual-core CPU running at 3.3GHz. That's rightthe CPU with the faster clock speed may actually be slower in practice, yet another example of why the number of cores matters so much.