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Ivy Bridge is simply a code name for an Intel architecture. Other examples are Sandy Bridge, Nehalem, Arrandale, Clarkdale, Conroe and a great many others.
The code name is never part of Intel's public name for a CPU but just a convenient way of designating a particular CPU family.
Ivy Bridge is Intel's current architecture. It supersedes Sandy Bridge, offering minor improvements in performance and power consumption. In fact the improvements are so minor that there's no point in getting IB if you already have SB. Intel's next family, which will offer more radical improvements will be called Haswell.
Ivy Bridge CPUs come in many shapes and forms. Intel's marketing department wants us to think of them in their convenient, oversimplified categories with i7 as high end, i5 as mainstream and i3 as entry. The reality is never so simple. To get the ideal CPU for your purposes you need to know what applications you want to use, and then decide whether you want a dual core or a quad core, whether you want a hyperthreaded quad core or not, whether you want to overclock and how much cache you want. It's a complicated picture but there are a few general rules.
- dual core i3 and i5s are fine for general use
- quad core i5s are best for gaming, particularly the K versions, because these allow unlimited overclocking
- quad core i7s are just as good as i5s (but no better) for gaming but are more expensive. For your extra money you're getting a larger cache and hyperthreading, which are useful for advanced applications that want a lot of CPU power like video editing, modelling and simulations
- hex core i7s are the high end and are overkill for gaming, but handy if you do a lot of work with advanced applications that need a lot of CPU power. At present, these exist only in Sandy Bridge flavours as Intel has not yet released the IB variant.
i5 2500k is a sandy bridge CPU. For many, it is the ultimate gaming CPU because it's quite cheap, just as good as an i7 for gaming and is very overclockable. It has been superseded by the IB i5 3570K. In i7 terms, the SB i7 2700K has recently been superseded by the IB i7 3770K.