It really depends how much you want to spend. I myself have owned the i5 2500k before, and it is a pretty decent card. Some stats for it can be found in the below spoiler:
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However, I currently own the AMD Phenom II X4 980 Black Edition, and it works just as well, if not better for about 50$ cheaper. Spoiler with the information below:
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Do take into consideration that the i5 is default on 3.3Ghz and can OC to 3.7Ghz, and the Phenom II X4 980 starts at 3.7Ghz, and I have no issues OC-ing it to 4Ghz. Really a point of preference between companies. Hope you can decide which one you would like. I also think it is unfair to classify the i7s and i5s, as the i7's are known for the 6-core ability. But you can still buy a 6-core Phenom II X6 1100T for 179$ MSRP, which is HUNDREDS of dollars cheaper. Spoiler with info for i7 990x Phenom II X6 1100T below:
AMD Phenom II X6 1100T: You must login or register to view this content. MSRP 179$
Intel i7 980x: You must login or register to view this content. MSRP 600$
Intel i7 990x: You must login or register to view this content. MSRP 1,000$
However, I currently own an AMD Phenom II X4 980 Black Edition, which I feel does the job the same, and has fairly similar specs, but is 50$ cheaper. Spoiler with stats: