The i7 950/960 are the previous generation. The only reason to choose them would be that you desperately need a machine with the highest possible memory bandwidth (they are triple channel instead of dual channel). They are, however, noticeably slower than the newer chips.
The i7 2600 and 2600K are very similar, and will perform almost identically at stock settings. The 2600K adds two things - it has more powerful integrated graphics (if you are not using a discrete video card), and it has an unlocked multiplier - you can over-clock the 2600K; you cannot over-clock the 2600 (to any significant level). If you want to run at stock clock, then get the 2600; if you want to over-clock, then get the 2600K.
The i5 2500K is almost as good as the i7 2600K, particularly for gaming. The i7 is hyper-threaded, and the i5 is not. If you are running things that benefit from hyper-threading, then you need the i7; if you aren't, then the i5 is rather cheaper.
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