The Intel Core i5-2500K processor is faster than anything in an economy computer and is the fastest choice before the prices really ramp up. When used with the Z68 chipset and fast memory, your applications will start and perform faster than your disks can supply data. You will need SSD to keep up.
The memory interface from the processor is the most important speed improvement. AMD set the way by building the memory interface into the processor. Intel copied AMD in the latest range of Intel processors. There is very little difference between the current options. Intel has a slightly faster internal connection. The 2500K has internal and external memory connections equivalent to the fastest current Intel desktop processor.
The fastest interface is the Intel LGA2011. LGA2011 currently is implemented to support only four sticks of memory. The 2500K uses the older LGA1155 and LGA1155 supports four sticks of memory. Next year LGA2011 will arrive with more memory connections and there will be a difference then, but only if you pay a thousand dollars or more extra for the hardware.
The clock speed of a processor is the second most important factor. The 2500K runs at 3.3 GHz with all four cores active, giving you a total of 13.2 GHz if your applications can use all the processors. My default set of applications use just 0.5 GHz when running. Startup uses more but rarely uses more than two cores.
When running just one core, the core can speed up to 3.7 GHz, a mode called turbo boost. Some operating systems work with turbo mode while others spread the work over all the cores and that stops turbo mode kicking in. For most types of work, spreading the work out is better than using turbo mode.
The most expensive current Intel desktop processor in the local shops is the Intel Core i7 3960X processor. The i7 3960X has exactly the same clock speed as the 2500K. For most activities, the 2500K will produce the same result.
The Intel Core i7 2600 has a slightly faster clock speed, 3.8 GHz, but costs a lot more. Comparing a system with the 2500 chip or the 2500K chip to either the 2600 or the 2600K, the money saved by buying the lower cost chip is enough to buy a fast SATA3 SSD instead of a medium speed SATA2 SSD.
Processors have internal cache memory to handle the difference in speed between the processor and the external memory. The 2500K has 6 MB of cache. The 2600K has 8 MB of cache. The extra cache in the 2600K is only of use if you run an application that floods memory with data and processes all the data. Movie rendering and CAD might make use of the extra cache. In that case the 15 MB cache in the i7 3960X might be more useful. You will also need the absolute fastest SSD and a few other improvements to make use of the extra cache.
Based on my tests for Web development and local database analysis, jumping from the 2500K to the 2600K will bring little benefit and not as much as spending the dollars on a faster SSD. For the extra cost of the i7 3960X, I could replace all my disks with the fastest SSD and get a bigger speed improvement.
The i5-2500K consumes 95 watts of power at maximum use. The fastest Intel desktop processors burn through 130 watts at peak usage. The difference is very little on your electricity bill, compared to the room lighting or the air-conditioning. What is a significant difference is the noise produced by the cooling fans in the computer. The hotter chips run the fans faster and the noise ramps up beyond annoying.
You can buy alternative cooling systems. There are plenty of very quiet options for the 95 watt 2500K. There are fewer options for the 130 watt CPUs. Based on listening to several options, the best coolers for the 95 watt LGA 1155 processors, including the 2500K, are so quiet at idle that you cannot hear them next to the other noises in a typical room. The coolers for the 130 watt processors never reach a low level of noise because the heat from an idle 130 watt processor is enough to keep the fans revolving at a noisy speed.
Built in graphics processor
Some Intel processors have built in graphics processing. Most are fast enough for browsing the Web but not for anything else. The Intel i5-2600 processor has a graphics processor fast enough to watch video from a DVD and most BluRay disks. The i5-2500K has a graphics processor running 50 percent faster than the i5-2600. You will find the graphics in the 2500K is fast enough for most uses.
When an application or game does need really fast graphics processors, you will find the application or game recommends a specific brand of graphics chip because their code is optimised for the chip. You will have to buy an expensive add-on card to get the right chip. If you are starting with an add-on graphics card, you can buy the cheaper 2500 instead of the 2500K. You will save $30 by buying the 2500 and that is an extra $30 to spend on the graphics card.
The fan supplied with the 2500K is quiet when idle but not as quiet as the best add-on fans. The supplied fan becomes noisy at maximum speed. A $42 add-on fan is almost noiseless at idle and quieter at all speeds. The bigger add-on fans often stay at idling speed when the supplied fan has to ramp up toward noisy. For $42, I prefer the big decrease in noise when you are working the computer full blast.
You will see the name
Sandy Bridge mentioned next to recent Intel processors. Sandy Bridge is just a new range for 2011. The slowest Sandy Bridge processors are slower than the medium speed processors in the previous range. When Sandy Bridge was first released, the fastest processors were not much faster than the previous fastest and were very expensive. After Sandy Bridge was released, Intel released some second generation Sandy Bridge processors and chipsets that really made extra speed affordable. If you use the latest Sandy Bridge processor with the latest Sandy Bridge chipset, you get significant extra speed for little extra cost.
For the faster i5 processors, you need the Z68 chipset. The Z68 chipset includes SATA3 so you can use the latest fast SSDs. A fast i5 with a fast SSD is a practical combination. Anything less starts to waste the extra speed of the Sandy Bridge range.