There's a lot you need to consider when you're choosing the proper "brain" for your computer, and there are a lot of CPU choices on the market. This guide will arm you with the basic information you need to make an informed choice.
This doesn't necessarily mean that you're doomed to a slow computer if you're watching every penny. As with most things in life, upgrading your CPU is really a matter of optimizing whatever resources you have. Making smart decisions about what you buy and why is therefore critical. Luckily, the process isn't complicated. By addressing just a few basic issues, you can find a chip that will net you surprisingly fast computer—regardless of what you have to spend.
Price
Usually, PC building or upgrading begins with asking, "What do I want to do?" Not this time. For all intents and purposes, you can perform all the same tasks with a $100 CPU that you can with a $500 one—the biggest difference is in how well you're able to do them. Deciding your budget is the crucial first step, for purposes of managing expectations as much as anything else. If you know right out of the gate that you may need 10 minutes to render a video rather than 20 seconds, you won't be disappointed when you discover your processor's limitations. Figure out the most you can spend on one component, and then see where that figure lies between (approximately) $100 and $1,000. The closer it is to the former, the slower it's probably going to be. There are exceptions to this we'll get to shortly, but it's a good rule of thumb.
AMD or Intel?
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, very few people will ever discover those differences in 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 $300, while the most expensive Intel chip runs $1,000. Does Intel deliver oomph for the money? In most cases, yes—but you won't necessarily be able to take advantage of it in every situation. Finding the right blend of performance and price for you may start with the CPU's manufacturer, but never assume it ends there.
Socket
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; for instance, online retailer NewEgg.com lists eight right now, and offers little clue of which are the most recent. AMD makes it easy: Its most current socket is the AM3, so if that's what your motherboard has, it can use all AM3 or AM2 or AM2+ CPUs. It's harder with Intel: Its mainstream processors use the LGA1156 socket, while its highest-performance models use the LGA1366, and the CPUs aren't interchangeable between them (or any of the previous socket types). 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. (Note: AMD and Intel are both scheduled to introduce new socket types with their 2011 processor platform updates, Fusion and Sandy Bridge: the AM3+ and the LGA1155, respectively. Fitting in with recent history, AMD's new socket will be backward compatible with older processors, and Intel's will not.)
Number of Cores
Until just a few years ago, this was an unheard-of attribute about processors—now, it's the one you'll want to focus on most. Almost all CPUs these days feature from two to six processing cores, which can all crunch data at the same time. Not every software program supports this capability, and not all the ones that do support it equally. But the apps that do—particularly intensive ones like photo and video editors—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 may find the investment worthwhile. Though there are some CPUs on the market—particularly older ones—that have only one processing core, we'd recommend avoiding these. Even dual-core CPUs are so ubiquitous and inexpensive now that you're actually making a smarter buying decision spending (a little) extra for two cores than going with just a single-core chip.
Note: Related to the number of cores a processor uses 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.) AMD-based CPUs turn out one thread per core; Intel processors, however, use a technology called Hyper-Threading that mimics multiple threads within cores (essentially sending information into the core and out of it at the same time), 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 application—with healthy performance boost (if not necessarily as much as twice the speed).
Clock Speed
Back when all CPUs were single-core, clock speed (or operating frequency) was king, and the surest way to determine how fast a piece of hardware you were getting. Though multiple processing cores are the most useful initial determiner now, 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 3-GHz 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 2 GHz (or two billion pulses per second). On multicore processors, the number of pulses stack up, so you can expect a four-core 3-GHz CPU to trounce a dual-core CPU running at 3.3 GHz. That's right—the CPU with the faster clock speed may actually be slower in practice, hence why the number of cores really matters.
Miscellany
The five qualities listed above are the most worth keeping in mind when you're shopping for a processor. But there are some other specifications that may not directly influence your buying decision, but that you'll want to be aware of just in case.
Manufacturing technology. The depth of the die from which the processor has been constructed. Over time, these tend to get smaller and more energy-efficient, and thus more powerful. The top-of-the-line CPUs on the market now use 32nm technology, and you'll still see many at 45nm.
64-bit support. Practically every CPU on the market today is a 64-bit model, which means it can process 64 bits (or eight bytes) of information at one time, as opposed to the 32 bits that were common until just a couple of years ago. This can mean significant performance improvements in 64-bit software (which is still ramping up in popularity and availability). Because so many of today's apps, from Windows to Photoshop, support 64-bit technology, you're better avoiding 32-bit processors on the off chance you come across them.
Cache. Of all the numbers you may encounter when perusing CPUs, cache can be the most confusing—in no small part because there are three different kinds, and their names are all very similar: L1, L2, and L3. The "L" stands for "level," and thinking about caches in those terms makes them easier to understand. L1 is the CPU's primary cache, the fastest section of its memory and the one most closely responsible for a processor's performance because it's where instructions are stored while they're waiting to be processed. L2 functions in almost exactly the same way, but instead of feeding instructions to the microprocessor itself, it feeds them to the L1 cache (and uses slower memory). The L3 cache feeds the L2, and is slower still. But in general, the more memory is in all of these, the more efficient the CPU will be.
TDP. Just like everything else inside your computer, processors require electricity. A CPU's Thermal Design Power (TDP) rating tells you how much you can expect it to require at full load. For most people, this isn't going to be a huge problem, but if you're already close to maxing out your power supply, it's something you'll want to think about: The fastest processor that uses the LGA1156 socket has a TDP of 95 watts, and the slowest uses 73 watts—but if you decide to splurge and upgrade to a Core i7-980X on the LGA1366 slot, its TDP is 130 watts. Many computer components will use more than this, so it's probably not something you'll need to worry about, but it's worth your attention.
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